# Heliophysics Seminars

The Heliophysics seminars are intended to allow guests and local members of the plasma physics community to present heliophysics related research and foster collaborations; to facilitate development of theoretical tools for understanding fundamental physical processes such as reconnection, turbulence, waves, and transport that control the dynamics in the context of the heliosphere; and to provide a forum to facilitate cross-fertilization between laboratory plasma physics, astrophysics, and heliospheric science.

### Upcoming

• Title: TBA
Prof. Steve Cranmer
#s1622, Tuesday, 28 Mar 2023, 2:00pm

### Past

• Frequency-resolved local measurements of phase-space energization **This will be a hybrid seminar on zoom and in person at the CCA in NYC**
E. Lichko, U. Chicago , abstract
[#s1619, 24 Jan 2023]
In order to disentangle the competing kinetic-scale energy dissipation processes that are intrinsic to space and astrophysical plasmas it is critical to be able to diagnose the energy transfer that is occurring locally in both time and space. A relatively recent technique to resolve the local rate of energy transfer between the fields and particles is the field-particle correlation (Klein & Howes APJL 2016), which has resolved local energy transfer at a single point in space for a large variety of systems and physical processes. This work details an updated version of the field-particle correlation that includes for the first time a breakdown of the energy transfer in frequency space, as well as time and velocity space. In addition to the increase in available information, this new method more cleanly separates magnitude and phase information of the signal, resulting in an improvement of the temporal resolution. This new method is applied to Gkeyll simulations of electron Landau damping as a proof of concept.
• Physical Regimes of Electrostatic Wave-Wave nonlinear interactions generated by an Electron Beam Propagation in Background Plasma
Haomin Sun [PPPL], abstract
[#s1611, 17 Jan 2023]
Electron-beam plasma interaction has long been a topic of great interest. Despite the success of Quasi-Linear (QL) theory and Weak Turbulence (WT) theory, their validities are limited by the requirement of sufficiently dense mode spectrum and small wave amplitude. In this paper, by performing a large number of high resolution two-dimensional (2D) particle-in-cell (PIC) simulations and using analytical theories, we extensively studied the collective processes of a mono-energetic electron beam emitted from a thermionic cathode propagating through a cold plasma. We confirm that initial two-stream instability between the beam and background cold electrons is saturated by wave trapping. Further evolution occurs due to strong wave-wave nonlinear processes. We show that the beam-plasma interaction can be classified into four different physical regimes in the parameter space for the plasma and beam parameters. The differences between the different regimes are analyzed in detail. For the first time, we identified a new regime in strong Langmuir turbulence featured by what we call Electron Modulational Instability (EMI) that could create a local Langmuir wave packet growing faster than ion frequency. Ions do not respond to EMI in the initial growing stage. On a longer timescale, the action of the ponderomotive force produces very strong ion density perturbations, and eventually the beam-plasma wave interaction stops being resonant due to strong ion density perturbations. Consequently, in this EMI regime, electron beam-plasma interaction is a periodic burst (intermittent) process. The beams are strongly scattered, and the Langmuir wave spectrum is significantly broadened, which gives rise to the strong heating of bulk electrons. Associated energy transfer from the beam to the background plasma electrons has been studied. A resulting kappa distribution and a energy spectrum observed in the strong turbulent regime are also discussed. This work was supported by the US Department of Energy, Office of Fusion Energy Science under contract # DE-AC02-09CH11466 as a part of the Princeton Collaborative Low Temperature Plasma Research Facility.
• Turbulent plasma wind tunnel studies on the Bryn Mawr Experiment (BMX) , Video
Prof. David Schaffner, Bryn Mawr
[#s1573, 19 Dec 2022]
• Intermittency and electron heating in kinetic-Alfven-wave turbulence video
Dr. Muni Zhou, IAS, abstract
[#s1572, 05 Dec 2022]
We report analytical and numerical investigations of sub-ion-scale turbulence in low-beta plasmas, focusing on the spectral properties of the fluctuations and electron heating. In the isothermal limit, the numerical results strongly support a description of the turbulence as a critically-balanced Kolmogorov-like cascade of kinetic Alfven wave fluctuations, as amended by Boldyrev & Perez (Astrophys. J. Lett. 758, L44 (2012)) to include intermittent effects. When the constraint of isothermality is removed (i.e., with the inclusion of electron kinetic physics), the energy spectrum is found to steepen due to electron Landau damping, which is enabled by the local weakening of advective nonlinearities around current sheets, and yields significant energy dissipation via a velocity-space cascade. The use of a Hermite-polynomial representation to express the velocity-space dependence of the electron distribution function allows us to obtain an analytical, lowest-order solution for the Hermite moments of the distribution, which is borne out by numerical simulations.
• Laboratory Study of Residual Energy Generation in Strong Alfvén Wave Interactions video
Dr. Mel Abler, Space Science Institute, abstract
[#s1571, 21 Nov 2022]
Plasmas confined by a dipole magnetic field exhibit interchange and entropy mode turbulence causing bursty intermittent transport of particles and energy [1]. On the Collisionless Terrella Experiment (CTX), this turbulence is dominated by low-frequency, long-wavelength modes with amplitudes and phases that vary chaotically in time [2]. We present a new paradigm for characterizing this turbulence by measuring the time-evolution of the fluctuation power spectrum and the instantaneous bispectrum using the continuous wavelet transform [3,4] and computing the statistical properties of turbulent wave kinetics. We observe that both the fluctuation power and the energy transfer by three-wave coupling between these fluctuations can be intermittent. When antenna are used to actively launch waves into the turbulence, the intermittency of the driven waves decreases, while the intermittency of other waves increases. Similarly, application of active feedback [5] to amplify the turbulence decreases the intermittency of the wave energy, while suppressing feedback increases this intermittency. Measurements based on this new paradigm show that the transfer of wave energy to larger and smaller scales in a turbulent plasma is not steady but occurs in short and intense bursts, analogous to the better-known short bursts of particle transport in magnetized plasma. [1] B.A. Grierson, et al, Phys Rev Lett 105, 205004 (2010). [2] B.A. Grierson, et al, Phys Plasmas 16, 055902 (2009). [3] C. Torrence and G.P. Compo, Bulletin of the American Meteorological Society, 61-78 (1998). [4] B.P. Van Milligen, et al, Phys Rev Lett 74, 395 (1995). [5] T.M. Roberts, et al, Phys Plasmas 22, 055702 (2015).
• SCALED SOLAR EJECTION EXPERIMENTS ON THE BIG RED BALL video
Dr. Rachel Young, University of Michigan, abstract
[#s1570, 14 Nov 2022]
At the Center for Laboratory Astrophysics at the University of Michigan, we specialize in laboratory experiments scaled to be relevant to astrophysics. How do we design laboratory experiments to answer questions about astrophysics? And what do we mean by “scaled” in terms of an experiment? This talk will explain how we go from an astrophysical system to a laboratory experiment by focusing on one project currently underway at the University of Michigan: studying solar ejections on the Big Red Ball machine at the University of Wisconsin. These are systems of magnetized plasma that the Sun ejects frequently and when they reach Earth cause geomagnetic storms. These storms are responsible for the aurora borealis (the northern lights) but they can also damage electrical equipment and cause power outages, and there is great interest in being able to understand and predict them better. Our Big Red Ball experiments are designed to address open questions about how these solar ejections react to external magnetic fields. Our experiments launch a compact torus of plasma (the scaled analog of the ejected coronal material) into background plasma inside the Big Red Ball (the scaled analog of the interplanetary medium, the diffuse plasma that fills the Solar System). I will discuss how the experiment was designed, how we went about “scaling” it, results of our first round of experiments, and plans for future work. This work is funded by the U.S. Department of Energy NNSA Center of Excellence under cooperative agreement number DE-NA0003869.
• Electron Energization during Electron-Only Magnetic Reconnection in PHASMA video
Dr. Peiyun Shi, West Virginia University, abstract
[#s1566, 07 Nov 2022]
Electron-only reconnection is a variant of magnetic reconnection without significant ion involvement. Such reconnection has recently been observed in the terrestrial magnetosheath and has received considerable attention due to its likely role in the energy cascade of turbulent magnetized plasmas. Different electron energization mechanisms usually favor electron acceleration along different directions and occur in different regions of reconnection. Therefore, spatially resolved electron velocity distribution function (EVDF) measurements along specific directions are required to elucidate the underlying electron energization mechanisms during electron-only reconnection. In the PHAse Space MApping (PHASMA) facility, a first-of-its-kind multi-dimensional incoherent Thomson scattering system enables 3D EVDF measurements by employing two injection paths and two collection paths. In this seminar, I will present 3D local electron heating measurement around the separatrix during reconnection. The measured heating corresponds to 70% of the incoming Poynting flux [Shi et al., Phys. Rev. Lett. 128, 025002 (2022)]. The electron temperature is clearly anisotropic, with the ratio between parallel and perpendicular electron temperature Te|| / Te⟂ ~ 1.5. This preferential parallel electron heating reveals the important role of the reconnecting electric field in electron energization for strong guide field reconnection. In addition, non-Maxwellian EVDFs, composed of a warm bulk population and a cold beam, were measured. Oppositely directed electron beams that appear on either side of the X-point are clear evidence of electron acceleration in magnetic reconnection. Interestingly, parallel EVDFs with either a flat top or beam component are also observed around two separatrices, reflecting the removal or enhancement of some electron portion of the electron velocity phase space. This agrees well with the field-particle correlation signatures reported in gyrokinetic numerical simulations [McCubbin et al., Phys. Plasmas 29, 052105 (2022)].
• Plasma-Physics Problems Operating an Accelerator in Space video
Joe Borovsky, Gian Luca Delzanno and Vadim Roytershteyn, abstract
[#s1563, 24 Oct 2022]
There is interest in operating a 1-MeV 1-kWatt electron accelerator on a spacecraft in the tenuous plasma of the Earth’s distant magnetosphere. The desire is to fire the electron beam into the atmospheric loss cone to optically excite the atmosphere at the magnetic footpoint of the accelerator. A NASA-funded engineering mission design was just completed for this mission concept. To bring the concept to reality, several plasma-physics problems had to be overcome. This talk will focus on 6 of those problems. (1) Designing an electron beam with a divergence smaller than the loss cone. (2) Knowing where the loss cone is located when there are finite-gyroradii effects. (3) Overcoming electron-beam-driven spacecraft charging. (4) Checking beam-plasma stability. (5) Examining whether ambient magnetospheric plasma waves will scatter the beam electrons. (6) Detecting the optical beamspot with ground-based imagers. Five out of six of these problems were difficult.
• A survey on recent problems in ionospheric plasma physics: Instabilities and parameter estimation video
Dr. Kike (Enrique) Rojas Villalba, Cornell, abstract
[#s1561, 10 Oct 2022]
The ionosphere is a weakly ionized plasma where electrostatic phenomena dominate. This plasma constantly interacts with the neutral atmosphere through particle collisions and dynamo-generated fields, and various free energy sources often drive it unstable, which can cause density irregularities and even storms. Nevertheless, despite its complexity and taking advantage of its response to radio waves, we can measure it with great precision using radars and other instruments. In this talk, I will present some of the ionospheric plasma physics projects I have been involved with recently, emphasizing potential commonalities with other plasma communities. First, I am going to describe the experimental characteristics of the collar-shaped echoes found at 150 km in the equatorial region, recent progress in understanding their origin, and how to use them to measure electron densities. Then, I will present some new results in modeling Farley-Buneman instabilities using fluid models and how these can capture various features measured with experiments and reproduced using PIC simulations. Next, I describe recent measurements of lower-hybrid waves in the topside of the ionosphere and a potential mechanism for their production. Finally, I will outline a method we are developing to estimate electron density distributions by fitting the computer estimations of radio wave refraction to local radar measurements. I will highlight open questions and potential solutions for each presented problem.
• Prof. David Schaffner, Bryn Mawr
[#s1471, 20 Jun 2022]
• Charging of dust grains in a plasma video
Prof. John Goree, University of Iowa, abstract
[#s1470, 06 Jun 2022]
A solid object immersed in a plasma collects electrons and ions unequally, so that its surface is at a floating potential, and the object as a whole gains an electric charge. This charging occurs for solid objects of all sizes, from moons to nanoparticles. When exposed to ultraviolet light or energetic electrons, the object can charge positively due to electron emission. On the other hand, a negative charge was almost always thought to develop in a low-temperature plasma, for a micron-size dust grain, until recently when it was discovered that a large positive charge can develop in an afterglow plasma. In this talk I review the basics of dust-grain charging, including for space-physics and industrial applications. I also present some recent experimental results with positive charging in an afterglow. Work supported by: Army Research Office under MURI Grant No. W911NF-18-1-0240, NASA-JPL subcontract 1672641, NSF grant 1740379-001, and U.S. Department of Energy Grant DE-SC0014566.
• Shocks Under the Microscope: Examining the Microphysics of Collisionless Shocks with High Resolution, Multi-point Space Plasma Measurements video
Prof. Katy Goodrich, University of West Virginia, abstract
[#s1469, 16 May 2022]
Collisionless shocks are an important and universal phenomenon in astrophysical plasmas. Shocks form when a supersonic plasma flow interacts with an impermeable obstacle. Examples of such interactions include galactic jets or supernova remnants interacting with the interstellar medium, or plasma wind from stars encountering stellar system bodies, such as planets, comets and moons. The shock performs the necessary function of converting kinetic energy to thermal energy, heating the originally supersonic plasma flow until its speed is reduced and it can flow around the obstacle. The energy conversion processes that take place inside collisionless shocks, however, are not not well established and have thus been a subject of interest for several decades. The closest and perhaps the most relevant collisionless shock to us humans, is the Earth’s bow shock. This shock forms 10-12 Earth radii upstream of our planet when supersonic plasma ejected from the sun, called the solar wind, meets the Earth’s intrinsic magnetosphere. The bow shock has been observed by multiple spacecraft such as ISEE, Cluster, WIND, and THEMIS over three decades. These missions have provided us with a wealth of information on plasma conditions both upstream and downstream of the shock, however, limited instrument capabilities have prevented us from resolving the physical processes active at the shock itself. The energy conversion processes active within shocks are expected to occur at primarily electron spatial and time scales (milliseconds to seconds), which up until recently were beyond the reach of our particle observations. While we can rely on highly time resolved magnetic and electric field measurements from these missions, they provide an incomplete view of the inner workings of the shock. Observations from the Magnetospheric Multiscale (MMS) mission (launched in 2014) provide particle observations on the order of 10s of milliseconds, providing an unprecedented opportunity to directly observe microscale processes inside collisionless shocks. In this talk, we’ll take a first look at the direct connection between electric and magnetic field signals with electron and ion dynamics in the Earth’s bow shock. We find that energy conversion can occur from multiple processes, some unexpected, within varied areas of the shock. The new dataset provided by MMS enables a new age of collisionless shock analysis as well as motivation for future space missions dedicated to the shock, signaling an exciting time for space physicists!
• Laboratory evidence for proton energization by collisionless shocks video
Dr. Weipeng Yao, Ecole Polytechnique, abstract
[#s1468, 09 May 2022]
Collisionless shocks are ubiquitously in the Universe and are held responsible for the production of non-thermal particles and high-energy radiation. Without particle collisions, theoretical works show that microscopic instabilities are able to mediate energy dissipation and allow for shock formation. Using our platform where we couple high-powerful lasers (JLF/Titan at LLNL and LULI2000) with high-strength magnetic fields, we have investigated the generation of magnetized collisionless shock and the associated particle energization [1]. We have diagnosed the plasma density, temperature, as well as the electromagnetic field structures and particle energization in the experiments, under various conditions of ambient plasma and B-field [2]. We have also modelled the formation and interpenetration of the shocks using both macroscopic hydrodynamic simulations and kinetic particle-in-cell simulations [3]. By varying the parameters of the expanding plasma launched in the ambient gas, as well as those of the background magnetic field, we investigate the bridge between the simulated dissipation mechanisms and observed particle energization, as will be reported here. [1] W. Yao, A. Fazzini, et al., Nat. Phys. 17, 1177–1182 (2021) [2] W. Yao, A. Fazzini, et al., MRE 7, 014402 (2021) [3] A. Fazzini, W. Yao, et al., arXiv:2202.03465 (2022)
• The Modifications of Collisionless Shock Hydrodynamics by Nonthermal Particles in Kinetic Simulations video
Prof. Colby Haggerty, University of Hawaii, abstract
[#s1467, 25 Apr 2022]
Earth’s bow shock, and collisionless plasma shocks more generally, are efficient accelerators of energetic, nonthermal particles. Shocks can deposit a significant fraction of their ram energy into non-thermal populations, yet despite this, much of our understanding of shock hydrodynamics is derived assuming the plasma behavies as a collisional fluid. With this in mind, we reconsider the theory of shock hydrodynamics including the effects of both the self generated, non-thermal, energetic population and the associated enhanced magnetic fields, amplified by the non-thermal particles. Using hybrid simulations of parallel collisionless shocks for a range of Mach numbers, it is shown that the non-thermal particles diffuse away from the shock in the downstream at an enhanced rate, increasing the compressibility and decreasing the speed of the shock. This nonlinear feedback process limits the fraction of shock energy deposited into the non-thermal particles and changes the slope of the associated power-law distribution function. These hydrodynamic modifications have important implications for our understanding of heliospheric shocks and how the sun and solar wind affect Earth’s magnetosphere as well as collisionless astrophysical shocks more generally.
• Enhanced Thomson scattering in super-Alfvenic, pulsed power driven magnetic reconnection experiments video
Dr. Lee Suttle, Imperial College London, abstract
[#s1466, 11 Apr 2022]
Pulsed-power driven exploding wire arrays have been used to demonstrate long-lived laboratory magnetic reconnection events in a quasi-2D geometry of colliding magnetized plasma flows [1]. The platform allows detailed measurements of reconnection layer structure and of the properties of the in- and out- flowing plasma, and provides control over the reconnecting plasma parameters (e.g. drive strength and radiative cooling time-scale) through the choice of the wire array material, enabling different non-idealized aspects of reconnection physics to be studied [2]. In this talk I will be discussing experiments carried out at the MAGPIE generator, using aluminum wire arrays which produce a reconnection layer with super-Alfvenic inflows (MA ~ 3, βdyn ~10). Thomson scattering (TS) measurements of the plasma temperatures and flow velocity components in the reconnection plane have shown a fast outflow of material along the layer consistent with the generalized Sweet-Parker model [3], and a preferential heating of ions inside the layer, reaching an energy partition of Ti ~ ZTe. Spitzer resistivity appears sufficient to balance Ohm’s law in this scenario, however it is unclear what mechanism is driving the ion heating. Further investigation was made using a TS geometry which is sensitive only to the out-of-plane component of plasma velocity – aligned with the reconnection electric field. This shows consistent plasma temperatures and electron drift velocities greater than the local sound speed of the plasma: a criterion for ion acoustic turbulence (IAT). An assessment of the spectrally integrated TS signal inside the current sheet also shows an enhancement in the scattered intensity, above the expected level of thermally-driven ion acoustic waves (IAWs), which is only present for current-aligned K vectors. However, the spectral shape of the observed IAWs is not consistent with previous observations of IAT [4]. We discuss and analyse potential implications of these measurements.
• Cross-scale energy transfer processes in weakly collisional plasmas video
Dr. Yan Yang, University of Delaware, abstract
[#s1462, 28 Mar 2022]
Space plasmas are frequently taken to be weakly collisional or collisionless. Therefore, an explicit form of viscous dissipation as in collisional (e.g., MHD) cases cannot be easily defined. A variety of approaches have attempted to characterize specific mechanisms (e.g., magnetic reconnection, wave-particle interaction and turbulent-driven intermittency) and to quantify the dissipation. However, the community has not come to a consensus solution applicable to all systems. Turbulence enters into space plasmas in many guises. The complexity and variability of the behavior of plasma turbulence is in large part due to the involvement of dynamics at many scales, ranging from macroscopic fluid to sub-electron scales. Observed turbulence in space is expected to involve cross-scale energy transfer and subsequent dissipation and heating. Instead of identifying specific mechanisms, we discuss how to disentangle multiscale properties, how plasma dynamics bridges multiple scales, what new ingredients are introduced in cross-scale transfer as models progress from fluid to kinetic, and how to identify key steps in energy transfer. This motivates several surrogates, arising from the energy transfer process, to estimate energy dissipation rate in weakly collisional plasmas. We investigate in detail the cross-scale energy transfer process and discuss unresolved issues (e.g., entropy and reversibility) that may be addressed by future studies. Where feasible, examples are given from MHD, Particle in Cell, and hybrid Vlasov-Maxwell simulations, and from Magnetospheric Multiscale (MMS) observations.
• Modeling CME-driven Solar/Stellar Energetic Particle Events at Solar-like Stars
Dr. Junxiang Hu, Center for Space Plasma and Aeronomic Research (CSPAR) / University of Alabama in Huntsville (UAH), abstract
[#s1460, 14 Mar 2022]
Coronal mass ejections (CMEs) and solar flares are arguably the two most energetic phenomena in the solar system. Large gradual solar energetic particle (SEP) events are primarily associated with coronal mass ejections (CME) driven shocks. The Discovery of frequent superflares on active cool stars opened a new avenue in understanding the properties of eruptive events and their impact on exoplanetary environments. Solar data suggest that CMEs should be associated with superflares on active solar-like planet hosts and produce Stellar Energetic Particle (StEP) events that could be orders of magnitude stronger than solar events. The improved Particle Acceleration and Transport in the Heliosphere (iPATH) model is a physics-based MHD SEP model developed at CSPAR. The iPATH model simulates diffusive shock acceleration (DSA) at CME-driven shocks and follows the subsequent transport of energetic particles through the inner heliosphere. Our recent work applies previous modeling efforts using iPATH for SEP events in the solar system to the stellar energetic particle events at young solar-like stars. We derive the scaling of StEP’s fluence and hardness of energy spectra with CME speed and associated flare energy. This study focuses on the very high energy (~hundreds of MeVs to GeVs) particles and their impact on erosion and chemistry of exoplanetary atmospheres. These results will have crucial implications for the prebiotic chemistry and expected biosignatures from atmospheres of early Earth and young rocky exoplanets, as well as the chemistry and isotopic composition of circumstellar disks around infant Suns.
• ParkerSolar Probe Observations of the Prevalence of Magnetic Reconnection in the near-Sun Heliospheric Current Sheet video
Tai Phan (UC Berkeley) and the PSP Team, abstract
[#s1433, 31 Jan 2022]
During its first eight orbits around the Sun, Parker Solar Probe (PSP) crossed the large-scale Heliospheric Current Sheet (HCS) multiple times and provided unprecedented detailed plasma and field observations of the near-Sun HCS. We report thecommon detections by PSP of reconnection exhaust signatures in the HCS at heliocentric distances of 16-107 solar radii. Both sunward and antisunward-directedreconnection exhausts were observed. In the sunward reconnection exhausts, PSP detected counter streaming strahl electrons, indicating that HCS reconnection resulted in the formation of closed magnetic field lines with both ends connected to the Sun. In the anti sunward exhausts, PSP observed dropouts of strahl electrons, consistent with the reconnected HCS field lines being disconnected from the Sun. Ion and electron signatures of the reconnection separatrix layers are also observed adjacent to some exhausts. The commondetection of reconnection in the HCS suggests that reconnection is almost always active in the HCS near the Sun. Furthermore, the occurrence of multiple long-duration partial crossings of the HCS suggests that HCS reconnection could produce chains of large bulges with spatial dimensions of up to several solar radii. The finding of the prevalence of reconnection in the HCS is somewhat surprising since PSP has revealed that the HCS is much thicker than the kinetic scales required for reconnection onset. Thus, the PSP findings suggest that large-scale dynamics either locally in the solar wind or within the coronal source of the HCS (e.g., at the tip of helmet streamers) plays a critical role in triggering reconnection onset.
• Exploring driven magnetic reconnection in the laboratory video
Dr. Joe Olson, University of Wisconsin, abstract
[#s1371, 13 Dec 2021]
In this talk, I'll discuss our recent work on the Terrestrial Reconnection Experiment (TREX) where magnetic reconnection is explored with asymmetric inflow conditions and in a configuration where the absolute rate of reconnection is set by an external drive. We find that magnetic pileup enhances the upstream magnetic field of the high-density inflow, leading to an increased upstream Alfvén speed, helping to lower the normalized reconnection rate to values expected from theoretical consideration. In addition, a shock interface between the far upstream supersonic plasma inflow and the region of magnetic flux pileup is observed, important to the overall force balance of the system, thereby demonstrating the role of shock formation for configurations including a supersonically driven inflow. Given the small system size relative to the ions, the observed normalized reconnection rate is consistent with a range of previous numerical results indicating a possible transition to "electron only" reconnection. These experimental results have also been corroborated through the use of PIC simulations utilizing the specialized TREX geometry and driving mechanism, showing similar scaling of the reconnection rate with system size. Finally, I'll touch on recent ongoing work to upgrade the reconnection drive system in an effort to reach more kinetic regimes of reconnection where effects such as electron pressure anisotropy are expected.
• Laboratory Simulation of Basic Space Plasma Phenomena video
Prof. Bill Amatucci, West Virginia University, abstract
[#s1370, 06 Dec 2021]
A rich variety of wave phenomena exist in virtually every region of the geospace environment. Understanding of the various wave signatures and driving mechanisms provided insight into the local conditions, not only at the instant that the waves are observed, but also into how the conditions may evolve in time. Comprehensive, controlled investigations of the detailed microphysics associated with these waves are difficult to accomplish through in situ methods alone. Space-based measurements typically offer a statistical picture, built up over long periods of time and under varying conditions. Consequently, properly scaled laboratory experiments performed under controlled, repeatable conditions can offer an important complementary approach to investigating the space plasma wave dynamics. We present an overview of several key wave modes found in the various regions with a particular emphasis on significant contributions of laboratory investigations toward the present understanding of the wave dynamics, validation of theoretical models, and interpretation of the in situ observations.
• An unexpected encounter in the inner Heliosphere: Solar Orbiter crossing through the ion tail of comet ATLAS. Structure of magnetic field draping and signatures from cometary pick-up ion waves close to the Sun. video
Prof. Lorenzo Matteini, Imperial College, abstract
[#s1369, 29 Nov 2021]
Soon after the launch of the ESA Solar Orbiter in February 2020 it was suggested that the spacecraft would have flown through the ion and dust tails of comet C/2019 Y4 (ATLAS) just after perihelion, while its nucleus was at 0.25 Astronomical Units from the Sun. The encounter is expected to have taken place when Orbiter was also close to its first perihelion at 0.5 AU, then offering us the first opportunity to study the tail of a comet inside 1AU. Moreover, comet ATLAS fragmented just before perihelion, making such an unusual near-Sun encounter even more interesting and a unique event. In this talk, I will briefly review some of the main plasma processes that characterise the interaction of an active comet with the surrounding solar wind, namely the draping of the interplanetary magnetic field around the nucleus, the pick-up of cometary ions and the generation of plasma waves by associated instabilities. Then, I will present joint observations from all in-situ instruments onboard Solar Orbiter about this event and will discuss identified signatures of magnetic field draping and ion-scale waves. Despite fragmentation and the fact that the encounter took place at a significant distance downstream of the nucleus, the overall structure observed at large scale is consistent with that of a comet magnetotail - 2 magnetic lobes of opposite polarity surrounding a central dense and almost unmagnetised plasma sheet - and qualitatively in agreement with past comet-tail crossings. Moreover, at smaller scale, we identified several intervals of ion waves likely associated to instabilities driven by cometary Oxygen ions picked-up by the solar wind and whose polarisation properties are in good agreement with predictions from numerical simulations. These results suggest that the spacecraft has indeed crossed the ion-tail of comet ATLAS at 0.5 AU and that events of this type could become more common in the near future thanks to the presence of Solar Orbiter, Parker Solar Probe and BepiColombo in the inner Heliosphere.
• Nonlinear coupling of whistler waves to oblique electrostatic turbulence enabled by cold plasmat
Dr. Vadim Roytershteyn, Space Science Institute, abstract
[#s1368, 15 Nov 2021]
Whistler waves play a major role in the dynamics of the Earth’s magnetosphere, where they are associated with local electron energization as well as precipitation in the form of aurora. Naturally occurring chorus waves are primarily generated by an instability driven by temperature anisotropy of hot (~keV) electrons which are injected during substorms. In addition, cold plasma populations (~eV) originating from ionosphere are often present and may in fact be the dominant population by density. In this presentation, I will describe our recent kinetic simulations and theory demonstrating that in the presence of cold populations whistler waves can excite oblique, short-wavelength electrostatic turbulence through a nonlinear process involving secondary drift instabilities. Depending on the parameters, the secondary modes are related to electron Bernstein and/or oblique whistler modes near the resonance cone. They lead to heating of the cold plasma, damping of the primary whistler waves, and may contribute to the excitation of the ﻿oblique chorus. The new mechanism can play a significant role in controlling amplitude of ﻿of whistlers in the regions of the Earth’s magnetosphere where cold background plasma of sufficient density is present. In addition to affecting the naturally occurring waves, this process may also be of importance to the efforts on so-called radiation belt remediation, i.e. attempts to enhance losses of dangerous energetic electrons from radiation belts by artificially injecting whistler waves.
• Space Physics in the PHAse SPace MApping (PHASMA) Experiment video
Prof. Earl Scime, West Virginia University , abstract
[#s1367, 01 Nov 2021]
A new experiment, called the PHAse Space MApping (PHASMA) experiment, features laser induced fluorescence diagnostics for ion measurements, Thomson scattering diagnostics for electron velocity distribution function measurements, and a microwave scattering system for turbulence measurements. PHASMA is designed to enable the direct measurement of ion and electron vdfs in space-relevant plasma phenomena including reconnection, shocks, and turbulence. To create the conditions necessary for different experimental regimes, PHASMA employs a 1 kW, steady-state helicon source capable of generating variable-density background hydrogen, helium, argon, krypton, and xenon plasmas with controllable plasma pressure (relative to the magnetic pressure), collisionality, and azimuthal flow shear. Reconnecting flux ropes arise through the merging of discharges from two pulsed plasma guns that operate in argon, helium, and hydrogen. In this talk I will describe results from two recent, space-relevant, experimental campaigns. The first campaign explored the stability of non-line-tied flux ropes. Similar boundary conditions exist for flus ropes observed in space and the solar corona. The measurements confirm the theoretically predicted stability threshold for the ropes and indicate that the m = 1 kink instability triggered for large currents in the flux rope drives an additional Alfvénic mode that propagates along the flux rope. In the second campaign, the merger of two flux ropes through electron-only reconnection is investigated at the kinetic, sub-gyroradius, scale. We find that the majority of the incoming magnetic energy appears as electron thermal energy and that Ohmic processes are unlikely to be responsible for the measured increase in electron enthalpy. The electron distribution function measurements include non-Maxwellian features, including beams that jet out from the X-point in both outflow directions. The electron beam speed scales with the local electron Alfvén speed.
• Dust in Space Plasmas: Investigations of Orbital Debris and Asteroid Evolution video
Prof. Christine Hartzell,University of Maryland , abstract
[#s1366, 25 Oct 2021]
The interaction of charged particles with the local plasma environment is of interest in a variety of space applications. This talk will focus on two specific applications: 1. the design of new technologies to detect orbital debris and 2. the evolution of asteroid surfaces. Small (sub-cm) orbital debris poses a serious hazard to functional spacecraft, but is very difficult to detect (and impossible to track) using conventional technologies. Our simulations predict that many of these debris objects will produce solitons in the ionospheric plasma environment. We will present the characteristics of the debris that are predicted to produce solitons and the characteristics of the resulting solitons. Detection of debris-produced solitons may provide a new technique to monitor the highly uncertain sub-cm debris environment. Our second topic concerns the removal of small particles from the surfaces of asteroids. The regolith of many small asteroids is coarse compared to other solar system bodies like the Moon. It has been hypothesized that small regolith particles may be removed from asteroids due to the interaction of the charged particles with the local plasma environment. We will present predictions of the sizes of particles that can be lofted from asteroid surfaces (considering two leading particle charging models) and the subsequent trajectories (including escape) of those particles. Electrostatic lofting, combined with solar radiation pressure, may be an active loss mechanism on asteroids.
• The helicity barrier: how low-frequency turbulence triggers high-frequency heating of the solar wind video
Prof. Jono Squire, University of Otago , abstract
[#s1365, 11 Oct 2021]
Low-frequency Alfvénic turbulence is a leading candidate to explain the heating of the solar corona and launching of the fast solar wind. A sufficiently energetic source of such motions is observed near the coronal base and in-situ measurements reveal that the solar wind is filled with Alfvénic fluctuations. However, a persistent difficulty with the scenario has been explaining the observed dominance of perpendicular ion heating, since theories predict a variety of outcomes, with electron heating dominating in the highly anisotropic, low-beta limit that seems most relevant to the low solar wind. Ion cyclotron wave (ICW) heating, in contrast, can readily explain the observed heating properties (including those of alpha particles and other minor ions); but, given the small parallel scale of ICWs, it has proved difficult to explain their source. In this work, using six-dimensional hybrid-kinetic simulations, we show how imbalanced Alfvénic turbulence can drive ICW heating of the solar wind. The effect is enabled by the recently discovered “helicity barrier,” which stops turbulent energy from cascading to scales below the ion gyroradius, thus inhibiting electron heating. Instead, the turbulent energy hits the “barrier” and is stuck, building up in time until the parallel scales decrease sufficiently to generate ICWs and perpendicular ion heating. The resulting turbulence bears detailed resemblance to a wide array of in-situ measurements from the solar wind, capturing the steep “transition range,” observed magnetic-helicity signatures, and key features of the ion distribution function. Based on the predicted dependence of the ion-to-electron heating ratio on imbalance, we suggest that fast- and slow-wind streams are driven by similar physical effects with the helicity barrier playing an important role. Preprint https://arxiv.org/abs/2109.03255
• Overview of plasma wave studies using the Basic Plasma Science Facility video
Troy Carter, UCLA, abstract, slides
[#s1354, 20 Sep 2021]
The Basic Plasma Science Facility (BaPSF) at UCLA is a US national collaborative research facility sponsored by DOE and NSF for studies of fundamental processes in magnetized plasmas. The centerpiece of the facility is the Large Plasma Device (LAPD), a 20m long, magnetized linear plasma device. LAPD has been utilized to study a number of fundamental processes, including: collisionless shocks, dispersion and damping of kinetic and inertial Alfvén waves, compressional Alfvén waves for ion-cyclotron range of frequencies heating, flux ropes and magnetic reconnection, three-wave interactions and parametric instabilities of Alfvén waves, turbulence and transport and interactions of energetic ions and electrons with plasma waves. An overview of research using the facility will be given, followed by a more detailed discussion of studies of the nonlinear physics of Alfvén waves and the physics of high power ICRF waves in LAPD. Recent experiments have resulted in the first laboratory observation of the parametric instability of shear Alfvén waves. Shear waves with sufficiently high ω/Ωc,i (> 0.6) and above a threshold wave amplitude are observed to decay into co-propagating daughter waves; one a shear Alfvén wave and the other a low-frequency quasimode. The observed process is similar to the modulational decay instability. Another series of experiments using LAPD have studied high power (~ 200kW) fast wave excitation (ω ∼ 2−10Ωci). Highlights of this work include documenting: the structure and scaling of RF sheaths, the formation of convective cells and associated density modification, and parasitic coupling to the slow mode in the low density plasma in front of the antenna.
• WIND Observations of Magnetic Reconnection Exhausts in the Solar Wind video
Dr. Stefan Eriksson, LASP University of Colorado, abstract
[#s1328, 30 Aug 2021]
The WIND satellite has been collecting magnetic fields at 92-ms and plasma moments at 3-s cadence in the pristine solar wind over nearly two full solar cycles, since it was launched on 1 Nov 1994. We report on the statistical properties in and around magnetic reconnection jets in the solar wind during a nearly 10-year period of continuous observations from 1 July 2004 to 31 Dec 2014. The conservative criteria of the external variability of the magnetic field and plasma velocity that we applied in the automatic surveys for jets across current sheets of variable durations resulted in a total of 4451 candidate exhaust events over this period. We have analyzed the Walen relation manually across all candidate events, and this exercise resulted in a subset of 3374 confirmed reconnection exhausts. This particular WIND data survey applied six running windows with time durations Δt=[12-s, 18-s, 2-m, 4-m, 10-m, 20-m] to the solar wind observations of magnetic field and bulk velocity. We present and discuss the histograms of several important parameters associated with these periods of current sheets with reconnection exhausts. We will also discuss some initial comparisons with the intensity of solar wind turbulence inside the exhausts and how this compares with the intensity of the external solar wind turbulence ahead of the current sheets.
• Concerted simulations and laser-driven laboratory experiments of fundamental astrophysical processes in turbulent magnetized plasmas
Prof. Petros Tzeferacos, University of Rochester, abstract
[#s1327, 23 Aug 2021]
I present an overview of the exciting fundamental science in magnetized astrophysical plasmas that the TDYNO team is accomplishing through concerted application of the FLASH code and laboratory plasma astrophysics experiments. Our international collaboration has conducted breakthrough experiments in the study of turbulent dynamo. This ubiquitous astrophysical mechanism is thought to be responsible for present-day magnetization of numerous celestial objects but had eluded laboratory plasma physicists for decades. The experiments have enabled us to explore dynamo in various regimes, providing us with novel insights and a new tool to validate or falsify our theoretical understanding. I will also describe how these experiments are enabling laboratory investigations of cosmic ray (CR) acceleration, the diffusive transport of extragalactic and ultra-high energy CRs, and the strong suppression of heat conduction in galaxy clusters.
• Flare simulations with the MURaM radiative MHD code video
Dr. Matthias Rempel High Altitude Observatory, National Center for Atmospheric Research, abstract
[#s1326, 16 Aug 2021]
Over the past few years, the MURaM radiative MHD code was expanded for its capability to simulate the coupled solar atmosphere from the upper convection zone into the lower solar corona. The code includes the essential physics to synthesize thermal emission ranging from the visible spectrum in the photosphere to EUV and soft X-ray from transition region and corona. A more sophisticated treatment of the chromosphere is currently under development. After a brief review of the code's capabilities and limitations I present a few recent examples of solar flare simulations computed with the MURaM code. Specifically, I present data inspired simulations that do capture 2 scenarios that have been proven to be flare productive in solar observations: emergence of new magnetic flux into an existing active region and compact collisional polarity inversion lines (cPILs) that result from the collision of sunspots. We compute synthetic observables including photospheric emission and coronal EUV and X-ray emission and study specifically observables that diagnose the pre-eruptive flux rope structure and highlight the location of reconnection sites leading to the destabilization of the flux system, which will be available from the Multi-slit Solar Explorer (MUSE), a mission undergoing a phase A study. In the simulated flares we find energy releases in the 5x1030 – 2x1031 erg range corresponding on the sun to flares of mid C to lower M class, leading to a multi-thermal plasma with temperatures exceeding 100 million K. While our current models cannot distinguish between thermal and non-thermal plasma, I discuss implications from the presence of a multi-thermal plasma for the interpretation of coronal observations.
• Using kinetic entropy to study energy conversion and dissipation in space plasmas video
Prof. Paul Cassak, University of West Virginia, abstract
[#s1325, 02 Aug 2021]
A recurring theme in the fundamental plasma physics of heliospheric, planetary, and astrophysical plasmas is the ultimate fate of large scale energy when it reaches small scales. This is certainly topical for magnetic reconnection, turbulence, and shocks which underly many physical processes of importance to these settings. In collisional plasmas, the fate of the energy is relatively straight-forward — it is irreversibly dissipated into heat at small-scale structures. In weakly collisional or collisionless plasmas, it is much less clear. This presentation will discuss our recent efforts to investigate entropy in the kinetic theory description as a possible tool. Recent theoretical developments have included a kinetic entropy-based non-Maxwellianity parameter and its applications. Numerically, we show kinetic entropy measures in fully-kinetic particle-in-cell (PIC) simulations of magnetic reconnection. We also show a comparison of a collection of measures that have been used to investigate kinetic-scale energy conversion in simulations of reconnection and turbulence using a suite of PIC, hybrid-Vlasov, and full Vlasov simulations. Finally, we show results of measuring kinetic entropy using the Magnetospheric Multiscale (MMS) satellites and discuss potential comparisons with laboratory plasma experiments.
• Can electromagnetic ion cyclotron waves propagate to the ground? 2D Full-wave simulations,video
Eun-Hwa Kim, PPPL, abstract
[#s1324, 19 Jul 2021]
This presentation numerically examines electromagnetic ion cyclotron (EMIC) wave propagation in the magnetosphere using the full-wave simulation tool, Petra-M. The Petra-M code is a state-of-the- art generic electromagnetic simulation tool for modeling RF wave propagation based on MFEM [http://mfem.org] and successfully examined wave properties by adopting realistic antenna geometry in tokamaks. This presentation adopts Earth’s dipole magnetic field geometry with a realistic density profile into the Petra-M and examines EMIC wave properties when waves have various wave normal angles (WNAs) and background heavy-ion densities. The EMIC waves are low-frequency waves typically in the Pc 1-2 frequency range that are excited below the proton gyrofrequency. The existing instability theories and ray tracing suggest that only left-hand polarized EMIC waves are generated near the magnetic equator and propagate along the field line toward the Earth. EMIC waves are predicted to reflect at the Buchsbaum resonance in the higher magnetic field region and not reach the ground. However, these results are inconsistent with observations. A1D full-wave analysis found that EMIC waves can tunnel through the evanescent region between cutoff and ion cyclotron resonance locations and reach the ground, but 1D modeling cannot include 2D magnetic curvature effects. 2D full-wave simulations enable us to overcome these shortcomings of ray tracing or 1D full-wave simulations using an approach that describes wave propagation, mode conversion, tunneling with 2D magnetic curvature effect for arbitrary plasma and magnetic field configurations. Previous 2D simulations using FW2D wave code showed excellent agreement with previous calculations, such as wave cutoff at the Buchsbaum resonance, polarization reversal, and mode conversion at the crossover locations. They also showed that equatorially generated EMIC waves could propagate into the inner or outer magnetosphere depending on the WNA, and thus suggested that WNAs could be one of the critical parameters to control EMIC wave propagation to the ground. However, since the previous work only focuses on wave properties near the ion cyclotron frequency, they did not provide a global picture of wave propagation. Here, we provide a global structure of the EMIC wave propagation from the source to the ground along the WNAs in various heavy ion compositions and densities. He-mode EMIC waves with small WNA cannot penetrate through the critical frequencies near the He gyrofrequencies; however, obliquely propagate He-mode EMIC waves can reach lower altitude and lower L-shell having right-handed polarization. Interestingly, the secondary mode conversion from the right- handed polarization EMIC waves to the linearly polarized Alfvenic wave occurs in the inner magnetosphere, and these waves can finally reach the ground.
• Alfvénic turbulence in the solar wind: An overview
Prof. Anna Tenerani, UT Austin, abstract, slides
[#s1321, 12 Jul 2021]
The solar wind carries a broadband of fluctuations in density, velocity and magnetic fields that, at the large scales, have been interpreted in terms of an ongoing magnetohydrodynamic turbulent cascade of Alfvénic fluctuations. These fluctuations carry sufficient energy to explain the non-adiabatic temperature gradients in the wind, and they may be a remnant of the flux responsible for coronal heating and solar wind acceleration close to the sun. However, a complete understanding of the origin and nonlinear evolution of Alfvénic turbulence in the solar wind still remains elusive. Parker Solar Probe (PSP), launched in 2018, will be the first spacecraft to fly into the sun’s corona, to within about 10 solar radii from the sun’s surface, with the goal to understand what heats the corona and accelerates the solar wind. Early measurements from PSP have already provided us with a wealth of data from regions of space never explored before. Measurements from the first orbits have shown the ubiquitous and persistent presence of the so-called switchbacks. These are magnetic field lines which are strongly perturbed to the point that they produce local inversions of the radial magnetic field. Switchbacks are embedded in the continuous flux of turbulent fluctuations emanating from the sun, and may be the remnant of coronal processes leading to solar wind formation – but their origin is still open to debate. In this seminar, we will review the main properties of Alfvénic fluctuations and switchbacks in the solar wind and discuss how their evolution is affected by parametric instabilities, kinetic effects and solar wind expansion. We will conclude by discussing the implications of our numerical and observational work for models of switchback generation and related open questions.
• Trigger Shy? A “Rosetta-Stone” Solar Eruption, video
Dr. Emily Mason, NASA Goddard, abstract, slides
[#s1279, 26 Apr 2021]
Coronal mass ejections, jets, prominence eruptions: solar eruptions are an active field with a broad range of accepted phenomena, and an even broader range of proposed mechanisms that cause the phenomena. This talk reports the observations of an event that connects the major eruption classes, and could provide a holistic explanation for all of them. The event originated in a filament channel overlying a circular polarity inversion line (PIL) and occurred on 2013 March 13 during the extended decay phase of the active region designated (sequentially) NOAA 12488/12501. This event was especially well-observed by multiple spacecraft and was seen to have the well-studied null-point topology. We analyze all aspects of the eruption using SDO AIA and HMI, STEREO-A, and SOHO LASCO imagery. One section of the filament undergoes a classic failed eruption with cool plasma subsequently draining onto the section that did not erupt, but a complex structured CME/jet is clearly observed by SOHO LASCO C2 shortly after the failed filament eruption. We describe in detail the long, slow buildup to eruption; the lack of an obvious trigger; and the immediate reappearance of the filament after the event. The unique mixture of major eruption properties that are observed in this event places severe constraints on the structure of the filament channel field and, consequently, on the possible eruption mechanism.
• Magnetohydrodynamics with embedded particle-in-cell model (MHD-EPIC) and its applications, video
Yuxi Chen, University of Michigan, abstract, slides
[#s1278, 19 Apr 2021]

It is challenging to capture kinetic phenomena in global simulations due to the significant difference between the kinetic scales and global scales. The magnetohydrodynamics with embedded particle-in-cell model (MHD-EPIC) is developed to incorporate kinetic physics into global simulations. It combines the physics capability of a particle-in-cell (PIC) code and the efficiency of an MHD model by coupling a semi-implicit PIC code with the global MHD model BATS-R-US. The PIC code is used to cover the regions where kinetic effects are important and the MHD model handles the rest part of the simulation domain.

We have improved the robustness of the PIC code by introducing the Gauss’s Law satisfying Energy Conserving Semi-Implicit Method (GL-ECSIM), which conserves energy and satisfies Gauss’s law numerically. Instead of modifying the electric field to satisfy Gauss’s law like the classical methods, we invented a new alternative approach: correcting the particle positions to satisfy the restriction. The simulation results demonstrate that the GL-ECSIM algorithm is accurate and robust. The capability of the MHD-EPIC model is further improved by using a new PIC code, the FLexible Exascale Kinetic Simulator (FLEKS). FLEKS allows PIC simulation domains of any shape so that it is more flexible to choose PIC regions in MHD-EPIC simulations. We have also designed particle resampling algorithms to further improve the accuracy and efficiency of FLEKS.

The MHD-EPIC model has been successfully applied to simulate planetary magnetospheres. We will present the simulation results of Earth’s and Mercury’s magnetospheres. The PIC region covers the dayside magnetopause in the simulation of Earth’s magnetosphere, and we studied the evolution of flux transfer events (FTEs) and compared the simulation results with observations. The MHD-EPIC model has also been applied to study the dawn-dusk asymmetry of Mercury’s magnetotail dynamics. It shows the asymmetries of the current sheet thickness, plasma density, reconnection site and reconnection jets.

• Using topology to locate the position where fully three dimensional reconnection occurs, video
Walter Gekelman, Department of Physics, UCLA , abstract, slides
[#s1277, 09 Apr 2021]
Magnetic flux ropes are bundles of twisted magnetic fields and their associated currents. They are common on the surface of the sun (and presumably all other stars) and are observed to have a large range of sizes and lifetimes. Two “kink” unstable ropes are generated in a background plasma in the Large Plasma Device at UCLA. When they collide fully 3D magnetic reconnection occurs. In a reconnection process magnetic energy is transformed to heat, energized particles and waves. In two dimensional magnetic reconnection, involving neutral sheets and magnetic islands it is generally a straightforward task to recognize reconnection sites when detailed data sets or simulations are available. In fully three dimensional reconnection their analogues can be challenging to identify. In this experiment the time dependent magnetic fields, plasma flows, electron temperature, plasma density, space charge and inductive electric fields were measured at over 42,000 spatial positions, 16,000 times steps requiring several million rope collision experiments. Magnetic field lines are followed in 3D and used to derive quasi-seperatrix layers (QSL), extended surfaces within which reconnection occurs. It turns out that QSL’s do not fully capture areas of reconnection. We have used additional topological quantities : the winding number (which measures the entanglement of pairs of field lines), magnetic twist, writhe and helicity to calculate a new quantity, the reconnection activity (RA). The RA identifies sub- regions of magnetic field lines which are reconnecting. It is demonstrated that the regions with the highest reconnective activity do not always coincide with the largest QSL signatures are, thus indicating this is a more complete methodology for quantifying reconnective activity than standard methods. This framework can serve as a model for reconnection analysis in future studies, in combination with established methods for identifying the specific form of reconnection once its location is established.

T. DeHass1 C. Prior2, A Yeates2

1 Tri Alpha Energy, Irvine Ca. 2 Durham University, United Kingdom

• The Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites Mission, video
Prof. Craig Kletzing, University of Iowa, abstract, slides
[#s1276, 02 Apr 2021]
The TRACERS mission consists of two identically instrumented satellites in the same low-Earth, sun-synchronous orbit with the spacecraft separated by 10-120s along the orbital track. The mission was selected for implementation in June, 2019 by NASA as a Small Explorer class investigation. The overarching mission goal of the TRACERS mission is: Connecting the magnetospheric cusp to the magnetopause – discovering how spatial or temporal variations in magnetic reconnection drive cusp dynamics. To address this goal, the TRACERS mission has three major scientific objectives: * Determine whether magnetopause reconnection is primarily spatially or temporally variable for a range of solar wind conditions. * For temporally varying reconnection, determine how the reconnection rate evolves. * Determine to what extent dynamic structures in the cusp are associated with temporal versus spatial reconnection. To accomplish this scientific research, TRACERS makes field and particle observations in the cusp in a Sun Synchronous Orbit at 500 km. Statistical analysis of the orbit shows that TRACERS will have more than 3250 cusp encounters in a one year mission lifetime. Well-proven instruments and good understanding of orbital characteristics allows for simple mission operations coupled with proven data analysis techniques backed by high-fidelity simulations. The instrumentation consists of ion and electron spectrometers, DC electric and magnetic field and AC wave measurements. We discuss the mission concept and relevance to space weather as well as presenting details of the spacecraft instruments, engineering challenges, and operational plan.
• Magnetic Reconnection Rate in Collisionless Plasmas, video
Prof. Yi-Hsin Liu, Dartmouth University, abstract, slides
[#s1275, 26 Mar 2021]
Magnetic reconnection is the process whereby the change in the magnetic field lines' connectivity allows for a rapid release of magnetic energy into the thermal and kinetic energy of the surrounding plasma. The magnitude of the reconnection electric field parallel to the reconnection x-line (where magnetic field lines break and rejoin) not only determines how fast reconnection processes magnetic flux, but can also be crucial for generating super-thermal particles. Observations and numerical simulations have revealed that collisionless magnetic reconnection in the steady-state tends to proceed with a normalized reconnection rate of an order of 0.1 in disparate systems. However, the explanation of fast reconnection remains an open question. In this talk, I will present a series of theory, modeling, and MMS (Magnetospheric Multiscale mission) observational studies on this issue. We propose that this value 0.1 is essentially an upper bound value constrained by the force-balance at the upstream and downstream regions, independent of the dissipation-scale physics, independent of the mechanism that localizes the x-line. The prediction from this model compares favorably to particle-in-cell simulations of magnetic reconnection in both the non-relativistic and extremely relativistic limits, from symmetric to asymmetric reconnection. Lately, we have included thermal pressure effects in our model to predict the rate in the high-beta limit. We also extend our study from 2D to the 3D system, studying the impact of a short x-line extent in the out-of-plane direction. Finally, we show that the maximum plausible reconnection rate could determine some of the 3D nature of magnetic reconnection, particularly the orientation of the x-line. These results could be interesting to researchers who study solar, magnetospheric, astrophysical, and laboratory plasmas.
• Parallel shock experiments on the Big Red Ball: Understanding the role of non-linear whistler waves, video
Doug Endrizzi, U. Wisconsin, abstract, slides
[#s1220, 12 Mar 2021]
Whistler waves are a common feature of space plasmas, notably present upstream of shocklets, SLAMS, and switchbacks in the solar wind. In the laboratory, they have been studied for at least 60 years, being easily generated from RF antennas, electron beams, and in pulsed power experiments. On the Big Red Ball vacuum vessel, strong whistler waves appeared in recent parallel shock experiments, where a supersonic high-density plasma piston was launched parallel to the magnetic field. These experiments explored a large range in plasma beta (from 0.04 up to above 1.0) and had diagnostic coverage allowing for 2D views of the interaction. Results will be presented showing the generation of dispersive whistler waves from an abrupt nonlinear ramp at the leading edge of the piston. Analysis of the whistler electric and magnetic fields will show how they mediate shock-like behavior in the cylindrical geometry.
• Ion Wakes: Modeling Dust Plasma Interactions, video
Prof. Lorin Matthews, Baylor University, abstract
[#s1252, 05 Mar 2021]
The interaction of an object within a streaming fluid is a phenomenon widely encountered in physics, spanning a range of length scales, from the familiar meter- to cm-sized wakes observed behind rocks in flowing streams (Fig. 1a), the kilometers-long wakes observed in cloud patterns as air flows past ocean islands (Fig. 1b), to the wakes produced in the bow shock of a speeding neutron star covering distances on the order of a parsecs (Fig. 1c). Plasma, a gas consisting of electrons, ions, and neutral molecules, can also be considered as a fluid. When plasma is moving with respect to an immersed object (such as a micron-sized dust grain), the object becomes charged and the trajectories of the ions in the plasma are altered as they flow past the charged body. Depending on flow velocity and the magnitude of the perturbing potential, ions can be focused into a region downstream of the object, creating an ion wakefield (Fig. 1d). Here we report results of coupled numerical models of the plasma discharge, ion wakefield and particle interactions in ground-based lab experiments and in microgravity experiments onboard the ISS.
• Alfven Wave Damping and Heating in the Solar Corona, video
Dr. Michael Hahn, Columbia, abstract, slides
[#s1219, 26 Feb 2021]
Understanding the mechanism by which the solar corona is heated to over a million Kelvin has been an unresolved problem in astrophysics for over 80 years. One theory is that energy is carried by Alfven waves into the corona where the waves are damped, thereby converting their energy into plasma heating. Using spectroscopic observations, we have found evidence that Alfven waves do carry enough energy for the heating and are indeed damped at low heights in the corona, as required by wave heating models. However, the physical processes that cause the wave damping are unknown. We are now investigating the cause of this damping through both observations and laboratory experiments. Recently, we studied intensity fluctuations in EUV images obtained by the Sun Watcher with Active Pixels (SWAP) instrument on the Proba2 satellite. These intensity fluctuations are proportional to density fluctuations, and show that density fluctuations grow in amplitude at heights similar to where the Alfven waves are damped. The density fluctuations change the local Alfven speed and are expected to cause reflection of the Alfven waves. Thus, the density fluctuations may help trap Alfven wave energy and promote dissipation through turbulence between the outward and reflected waves. We have also been carrying out laboratory experiments using the Large Plasma Device at the University of California Los Angeles. There, we have studied the propagation of Alfven waves through Alfven-speed gradients similar to those in the corona. Our results confirm that the transmission of Alfven wave energy is significantly reduced by the gradient. Surprisingly, though, we have not observed any reflection of the Alfven waves, which is the mechanism predicted by theory to be responsible for the reduced transmittance.
• Flux Ropes, Turbulence, and Collisionless Perpendicular Shock Waves: High Plasma Beta Case, video
Prof. Gary Zank, U. Alabama, abstract, slides
[#s1218, 12 Feb 2021]
With the onset of solar maximum and the likely increased prevalence of interplanetary shock waves, Parker Solar Probe is likely to observe numerous shocks in the next few years. An outstanding question that has received surprisingly little attention has been how turbulence interacts with collisionless shock waves. Turbulence in the supersonic solar wind is described frequently as a superposition of a majority 2D and a minority slab component. We formulate a collisional perpendicular shock-turbulence transmission problem in a way that enables investigation of the interaction and transmission of quasi-perpendicular fluctuations such as magnetic flux ropes/islands and vortices as well as entropy and acoustic modes in the large plasma beta regime. We focus on the transmission of an upstream spectrum of these modes, finding that the downstream spectral amplitude is typically increased significantly (a factor of 10 or more), and that the upstream spectral index of the inertial range, and indeed the general spectral shape, is unchanged for the downstream magnetic variance, kinetic energy, and density variance. A comparison of the theoretically predicted downstream magnetic variance, kinetic energy, and destiny variance spectra with those observed at 1 au, 5 au, and 84 au by Wind, Ulysses, and Voyager 2 shows excellent agreement. The overall theoretically predicted characteristics of the transmission of turbulence across shocks observed in the solar wind appear to be largely consistent with recent observational studies by Pitna et al. 2016, Pitna et al. 2020, and Borovsky 2020.
• Dynamic, Astrophysically- and Solar-Relevant MHD Plasma Experiments, video
Paul M. Bellan, Caltech, abstract
[#s1217, 05 Feb 2021]
Dynamics relevant to solar and astrophysical plasmas is being investigated using MHD-regime lab experiments. High-speed imaging resolves sub-Alfven time scales and reveals unexpected phenomena. Images show that highly collimated MHD-driven, finite-beta plasma flows occur and can be considered a lab ‘replica’ of an astrophysical jet. Having both axial and azimuthal magnetic fields, the jet can be considered to be a flux rope, i.e., a plasma-confining flux tube with embedded helical magnetic field. The jet velocity is proportional to axial electric current and the jet flows axially from where the flux tube radius is small to where it is large. Jet stagnation compresses embedded azimuthal magnetic flux resulting in self-collimation. An expanding solar coronal loop is effectively two jets pointing at each other carrying a common current and axial flux. Jets kink when they breach the Kruskal-Shafranov stability limit. Lateral acceleration of a sufficiently strong kink produces a substantial effective gravity that provides the environment for a spontaneously developing fine-scale, fast Rayleigh-Taylor instability. This instability involves ‘heavy’ plasma interchanging with ‘light’ plasma in a series of ripples that choke the current channel to less than the ion skin depth. This cascade from the MHD scale to the ion skin depth scale can result in a fast magnetic reconnection whereby the jet breaks off. Several distinct abrupt phenomena are observed when this happens. These include: radiation of a hard X-ray burst, emission of a whistler wave burst, localized EUV emission, and dimming of visible light
• Using magnetic fields and microgravity to explore the physics of dusty plasmas, video
Prof. Ed Thomas, Auburn, abstract
[#s1171, 18 Dec 2020]
Over the last three decades plasma scientists have learned how to control a new type of plasma system known as a “complex” or “dusty” plasma. These are four-component plasma systems that consist of electrons, ions, neutral atoms, and charged, solid, nanometer- to micrometer-sized particles. The presence of these microparticles allow us to “tune” the plasma to have solid-like, fluid-like, or gas-like properties. This means that dusty plasmas are not just a fourth state of matter – they can take on the properties of all four states of matter. From star-forming regions to planetary rings to fusion experiments, charged microparticles can be found in many naturally occurring and man-made plasma systems. Therefore, understanding the physics of dusty plasmas can provide new insights into a broad range of astrophysical and technological problems. This presentation introduces the physical properties of dusty plasmas – focusing on how the small charge-to-mass ratio of the charged microparticles gives rise to many of the characteristics of the system. In particular, dusty plasmas can be used to study a variety of processes in non-equilibrium or dissipative systems such as self-organization and energy cascade as well as a variety of transport and instability mechanisms. This presentation will discuss results from our studies of dusty plasmas in high (B ≥ 1 T) magnetic fields using the Magnetized Dusty Plasma Experiment (MDPX) device at Auburn University and in microgravity experiments using the Plasmakristall-4 (PK-4) laboratory on the International Space Station.
• Solar Wind Turbulence: in-situ observations from magneto-fluid to kinetic plasma scales, video
Dr. Olga Alexandrova, Observatorie de Paris, LESIA, abstract, slides
[#s1170, 04 Dec 2020]
This seminar is devoted to solar wind turbulence from MHD to kinetic plasma scales. Solar wind turbulence was mostly studied at MHD scales: there, magnetic fluctuations follow the Kolmogorov spectrum. The fluctuations are mostly incompressible and they have non-Gaussian statistics (intermittency), due to the presence of coherent structures in the form of current sheets, as it is widely accepted. Kinetic range of scales is less known and the subject of debates. We study the transition from Kolmogorov inertial range to small kinetic scales with a number of space missions. It becomes evident that if at ion scales (100-1000 km) turbulent spectra are variable, at smaller scales they follow a general shape. Thanks to Cluster/STAFF, the most sensitive instrument to measure magnetic fluctuations by today, we could resolve electron scales (1 km, at 1 AU) and smaller (up to 300 m) and show that the end of the electromagnetic turbulent cascade happens at electron Larmor radius scale, i.e., we could establish the dissipation scale in collisionless plasma. Recently, we have confirmed these results closer to the Sun (at 0.3 AU). Furthermore, we show that intermittency is not only related to current sheets, but also to cylindrical magnetic vortices, which are present within the inertial range as well as in the kinetic range. This result is in conflict with the classical picture of turbulence at kinetic scales, consisting of a mixture of kinetic Alfven waves. The dissipation of these waves via Landau damping may explain the turbulent dissipation. How does this picture change if turbulence is not only a mixture of waves but also filled with coherent structures such as magnetic vortices? These vortices seem to be an important ingredient in other instances, such as astrophysical shocks: for example, they are observed downstream of Earth's and Saturn's bow-shocks. With the new data of Parker Solar Probe and Solar Orbiter we hope to study these vortices closer to the Sun to better understand their origin, stability and interaction with charged particles
• Relativistic electron-ion shocks in gamma-ray bursts: What about pair loading?
Dr. Daniel Groselj, Columbia, abstract
[#s1169, 20 Nov 2020]
In the external gamma-ray burst (GRB) collisionless shocks, intense radiation gives rise to abundant production of electron-positron pairs in two-photon collisions. The upstream medium is thus transformed into a mixed composition electron-ion-positron plasma. How exactly the pair loading affects the structure, and the resulting particle acceleration, of external GRB shocks is largely unknown. Here, I present a set of first-principles kinetic simulations to elucidate the microscale physics of relativistic pair-plasma-loaded, weakly magnetized shocks. I will show that even moderate changes in the plasma composition can significantly impact the shock dynamics. More specifically, I will demonstrate that (i) the transition from a Weibel-mediated to a magnetized type shock becomes a function of the pair loading factor Z, (ii) the energy fraction transferred from ions to pairs is only weakly dependent on Z, and (iii) pair-loaded shocks are efficient particle accelerators in the limit of vanishing magnetization. These findings have important implications for the modeling of the early afterglow emission of GRBs.
• Experimental evidence of detached bow shock formation in the interaction of a laser-produced plasma with a magnetized obstacle
Dr. Joseph Levesque, LANL, abstract
[#s1179, 06 Nov 2020]
The magnetic field produced by planets with active dynamos, like the Earth, can exert sufficient pressure to oppose supersonic stellar wind plasmas, leading to a standing bow shock upstream of the pressure-balance surface, known as the magnetopause. Scaled laboratory experiments studying the interaction of an inflowing solar wind analog with a strong, external magnetic field can provide another way to study magnetospheric physics and complement existing models. In this talk I present experimental evidence of the formation of a magnetized bow shock in the interaction of a supersonic, super-Alfvenic plasma with a strongly magnetized obstacle at the OMEGA laser facility. The plasma source for these experiments is generated by the simultaneous laser-irradiation of two thin carbon discs, the resulting counter-propagating plasma plumes collide and subsequently expand outward toward the magnetized obstacle, which is a thin, current-carrying wire. We measure the plasma number density in the interaction region using Spatially resolved, optical Thomson scattering, from which we infer the presence of what appears to be a fast magnetosonic shock far upstream of the obstacle. Proton images additionally provide a measurement of large-scale features of the magnetic field topology based on proton deflections, and further suggest the formation of a bow shock by an inferred compression of the magnetic field in our system. From these images we determine the shock standoff distance and analyze the evolution of the bow shock for two applied field strengths.
• The Earth's Ion Foreshock: A natural laboratory for ion beam-generated waves and non-linear wave processes
Dr. Seth Dorfman, SRI, abstract
[#s1168, 30 Oct 2020]
Waves generated by accelerated particles are important throughout our heliosphere. These particles often gain their energy at shocks via Fermi acceleration. At the Earth's bow shock, this mechanism accelerates ion beams back into the solar wind; the beams can then generate ultra low frequency (ULF) waves below the ion cyclotron frequency via an ion-ion right hand resonant instability. These ULF waves influence the shock structure and particle acceleration, lead to coherent structures in the magnetosheath, and are ideal for non-linear interaction studies relevant to turbulence. We report the first satellite measurement of the ultralow frequency (ULF) wave growth rate in the upstream region of the Earth's bow shock [1]. This is made possible by employing the two ARTEMIS spacecraft orbiting the moon at ∼60 Earth radii from Earth to characterize crescent-shaped reflected ion beams and relatively monochromatic ULF waves. Using ARTEMIS data, the ULF wave growth rate is estimated and found to fall within dispersion solver predictions during the initial growth time. Observed frequencies and wave numbers are within the predicted range. Other ULF wave properties such as the phase speed, obliquity, and polarization are consistent with expectations from resonant beam instability theory and prior satellite measurements. Building on this result, new work is underway to determine the statistical properties of the ULF waves at the location of ARTEMIS and make comparisons with the global hybrid-Vlasov code Vlasiator. For example, an analysis of all foreshock events observed by ARTEMIS from 2011-2019 shows a clear preference for the left-hand polarized waves (in the spacecraft frame) expected from the ion-ion right hand resonant instability. However, unlike a 2.5-D Vlasiator simulation in which the waves are entirety left-hand polarized, ARTEMIS data shows a significant right-hand component that is unlikely to be directly generated by the ion beam. This component may therefore result from non-linear processes that are 3-D in nature with potential applications to turbulence and dissipation in the heliosphere.
• Understanding Our Heliospheric Shield: Laying the Groundwork to Predict Habitable Astrospheres
Prof. Merav Opher, Boston University, abstract
[#s1167, 23 Oct 2020]
The heliosphere is an immense shield that protects the solar system from harsh, galactic radiation. This radiation affects not only life on Earth, but human space exploration as well. In order to understand the evolution of the heliosphere’s shielding properties, we need to understand its structure and large-scale dynamics. The heliosphere is a template for all other astrospheres, enabling predictions about the conditions necessary to create habitable planets. Space science is at a pivotal point in generating new understandings of the heliosphere due to the flood of new in situ data from the Voyager 1 (V1), Voyager 2 (V2), and New Horizon spacecraft, combined with the energetic neutral atom (ENA) maps generated by IBEX and Cassini. I this talk I will review some of the most pressing aspects that need understanding in the heliosphere. Among them, the shape of the heliosphere. The canonical view of the structure of the heliosphere is that it has a long comet-like tail. This view is not universally accepted and there is vigorous debate as to whether it possesses a long comet-like structure, is bubble shaped, or is “croissant”-like, a debate that is driven by observations and modeling. Opher et al. (2015) suggest a heliosphere with two lobes, described as “croissant”-like. An extension of the single ion global 3D MHD model that treats PUIs created in the supersonic solar wind as a fluid separate and distinct from the thermal solar wind plasma yields a heliosphere that is reduced in size and rounder in shape (Opher et al. 2020). In contrast, Izmodenov et al. 2020 argue that a long/extended tail confines the plasma. One direct way to probe the structure of the tail is through energetic neutral atom (ENA) maps. ENA images of the tail by Interstellar Boundary Explorer (IBEX) at energies of 0.5-6keV exhibit a multi-lobe structure. These lobes are attributed to signatures of slow and fast wind within the extended heliospheric tail as part of the 11-year solar cycle (McComas et al. 2013; Zirnstein et al. 2017). Higher energy ENA observations (>5.2 keV) from the Cassini spacecraft, in conjunction with >28 keV in-situ ions from V1&2/LECP (Dialynas et al. 2017), in contrast, support the interpretation of bubble-like heliosphere. Regardless of the shape of the heliotail, there is an agreement between models that the solar magnetic field in the inner heliosheath (IHS) possesses a “slinky-like” structure (Opher et al. 2015; Pogorelov et al. 2015; Izmodenov et al. 2015) that helps confine the plasma in the IHS. I will review some of the recent discoveries and challenges as part of the recently funded NASA Science Center SHIELD (Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics).
• Kinetic physics of the electrons in the solar wind
Prof. Daniel Verscharen, University College London, abstract
[#s1118, 09 Oct 2020]
The electron distribution function in the solar wind consists of three main populations: a thermal core, a suprathermal, quasi-isotropic halo, and a field-aligned beam called “strahl”. In contrast to the protons, the electrons are a sub-sonic particle population, and due to their small mass, they contribute little to the overall momentum flux of the solar wind. However, their unique kinetic properties supply the solar wind with a significant heat flux. We investigate the regulation of this heat flux by kinetic microinstabilities. I will present a mathematical framework for the description of electron-driven instabilities and discuss the associated physical mechanisms. We find that an instability of the oblique fast-magnetosonic/whistler (FM/W) mode is the best candidate for a microinstability that regulates the strahl heat flux by scattering strahl electrons into the halo population, consistent with spacecraft measurements. We derive approximate analytic expressions for the FM/W instability thresholds and confirm their accuracy through comparison with numerical solutions to the hot-plasma dispersion relation. The comparison of our theoretical results with a large statistical dataset from the Wind spacecraft confirms the relevance of the oblique FM/W instability for the solar wind. In addition, we find a good agreement between our theoretical results and numerical solutions to the quasilinear diffusion equation. I will present our results in the context of the latest measurements from Parker Solar Probe and Solar Orbiter.
• Interplay among Arched Plasma Eruptions, Global Oscillations, and Broad Spectra of Alfvén Waves
Dr. Shreekrishna Tripathi, UCLA, abstract
[#s1117, 18 Sep 2020]
Arched magnetized structures that carry electrical current ubiquitously exist in solar and heliospheric plasmas. Varieties of plasma waves and current-driven instabilities (e.g., fast waves, kink, sausage, and Kelvin-Helmholtz instabilities) have been at the forefront of contemporary research in solar and heliospheric physics. After introducing key concepts related to eruptive processes in solar physics, results from a laboratory experiment on arched magnetized plasmas (plasma β ≈ 10-3, Lundquist number ≈ 102–105, plasma radius/ion-gyroradius ≈ 20, B ≈ 1000 Gauss at footpoints, 1/2 Hz repetition rate) will be presented. The arched plasma is created using a lanthanum hexaboride plasma source and it evolves in an ambient magnetized plasma produced by another source. The plasma and wave parameters are recorded with a good resolution using movable Langmuir and three-axis magnetic-loop probes in 3D. Images of the plasma are recorded using a CCD camera. In the upgraded experiment, the main focus is on the direct measurement of propagation and damping characteristics of global kink-mode oscillations and fast waves. These waves are frequently observed after eruptive events on the Sun. Recent results reveal fascinating interplay among global oscillations of the arched plasma and fast waves. Transverse gradients in Alfvén speed across the arched plasma have been observed to excite a broad spectra of fast Alfvén waves that carry away energy from large scale oscillations in the arched plasma. These observations are consistent with predictions of the phase mixing of fast waves in an inhomogeneous magnetized plasma that effectively enhances damping of large scale oscillations. Phase-mixing of these waves is likely to play important role in affecting the energetic of the solar atmosphere.
• Study the Alfvén-wave acceleration of auroral electrons in the laboratory using field-particle correlations
Prof. Jim Schroeder, Wheaton College, abstract
[#s1119, 11 Sep 2020]
The acceleration of auroral electrons is primarily attributed to quasistatic field-aligned currents in the magnetosphere. However, dispersive Alfvén waves in inertial plasmas (vA > vte) have an electric field parallel to B0 and are frequently detected in the auroral magnetosphere traveling earthward with sufficient Poynting flux to produce auroras. Test particle simulations in relevant plasma conditions show inertial Alfvén waves can resonantly accelerate electrons to auroral energies. Satellite surveys find that inertial Alfvén waves deposit an amount of energy in the lower magnetosphere capable of accounting for one-third of all auroral luminosity. Despite these results supporting the hypothesis that inertial Alfvén waves accelerate a significant fraction of auroral electrons, the limitations of spacecraft data have so far prevented direct evidence of the acceleration process from being found. Laboratory experiments in UCLA’s Large Plasma Device seek to provide insight by launching inertial Alfvén waves and simultaneously measuring the parallel electron velocity distribution. The electron distribution is measured using wave absorption, a technique where a small-amplitude probe wave is absorbed in proportion to the number of resonant electrons. Alfvénic perturbations to the electron distribution have been detected, and, using a field-particle correlation, energy transfer to electrons from the launched Alfvén waves has been found. Experimental results are interpreted using kinetic theory and numerical simulations.
• A Laboratory Model for Magnetized Stellar Winds
Dr. Ethan Peterson, MIT, abstract
[#s1116, 21 Aug 2020]
Eugene Parker developed the first theory of how the solar wind interacts with the dynamo-generated magnetic field of the Sun. He showed that the wind carries the magnetic field lines away from the star, while their footpoints are frozen into the corona and twisted into an Archimedean spiral by stellar rotation. The resulting magnetic topology is now known as the Parker spiral and is the largest magnetic structure in the heliosphere. The transition between magnetic field co-rotating with a star and the field advected by the wind is thought to occur near the so-called Alfv\'en surface - where inertial forces in the wind can stretch and bend the magnetic field. According to the governing equations of magnetohydrodynamics, this transition in a magnetic field like the Sun's is singular in nature and therefore suspected to be highly dynamic. However, this region has yet to be observed in-situ by spacecraft or in the laboratory, but is presently the primary focus of the Parker Solar Probe mission. Here we show, in a synergistic approach to studying solar wind dynamics, that the large-scale magnetic topology of the Parker spiral can also be created and studied in the laboratory. By generating a rotating magnetosphere with Alfv\'enic flows, magnetic field lines are advected into an Archimedean spiral, giving rise to a dynamic current sheet that undergoes magnetic reconnection and plasmoid ejection. These plasmoids are born at the tip of the streamer cusp, driven by non-equilibrium pressure gradients, and carry blobs of plasma outwards at super-Alfv\'enic speeds, mimicking the observed dynamics of coronal helmet streamers. Further more, a simple heuristic model based on a critical plasmoid length scale and sonic expansion time is presented. This model explains the frequencies observed in the experiment and simulations (10s of KHz) and is consistent with the 90 minute plasmoid ejection period of full-scale coronal streamers as observed by the LASCO and SECCHI instrument suites.
• Constructing a Rosetta Stone for Plasma Heating and Particle Acceleration in Kinetic Plasma Physics
Prof. Gregory Howes, University of Iowa, abstract
[#s1115, 14 Aug 2020]
The general question of how plasmas are heated and particles accelerated underlies many key challenges at the frontier of heliophysics and astrophysics, including solar coronal heating, particle acceleration in solar flares and supernova remnants, and auroral electron acceleration. The hot and diffuse plasmas in many space and astrophysical environments lead to weakly collisional conditions, so plasma kinetic theory is essential to understand both how particles are energized and whether that leads to heating of the bulk plasma or the directed energization of accelerated particles. The field-particle correlation technique is an innovative method to understand how the electromagnetic fields energize particles in weakly collisional plasmas, yielding a velocity-space signature that is characteristic of a given mechanism of energization. These signatures can be used both to distinguish and identify the mechanism at play and to determine the net rate of particle energization. I will present the construction of a "Rosetta Stone" of these velocity-space signatures that can be used to identify the mechanisms of energization in kinetic plasma turbulence, collisionless magnetic reconnection, and collisionless shocks.
• Proton temperature anisotropy, Alfven waves, and the turbulence heating problem in the solar wind
Prof. Robert Wicks, University of Northumbria, abstract
[#s1114, 07 Aug 2020]
Over the last 10 years many different studies have shown different and related forms of anisotropy about the magnetic field in the solar wind plasma. Protons have anisotropic temperature, the turbulent fluctuations have different amplitudes, polarisations, and frequency-dependent scaling, and instabilities and coherent waves propagate, grow and damp at different rates depending on their relative direction to the magnetic field. The big problem with this is that measurements made by single spacecraft rely on the solar wind flow to provide different measurement directions relative to the magnetic field. This means that our perception of what is happening is heavily biased by what occurs in the direction of flow of the plasma (radially away from the Sun). In this talk, I will review results investigating anisotropy and describe a novel method to leverage the Taylor hypothesis to identify the field-parallel and -perpendicular components of wavevectors measured by a single spacecraft. Comparing these results to proton temperature anisotropy then allows us to show that instabilities growing in the field-parallel direction are primarily cyclotron waves and associated with strong proton beams, and in the perpendicular direction are firehose instabilities (although these are rarer). Furthermore, we can associate the polarisation of the magentic field waves routinely observed close to the gyrofrequency to the different branches of the Alfven wave dispersion relation, confirming that the modes are at least somewhat similar to linear waves with left-handed polarisation in the parallel direction and right-handed in the perpendicular. When we sample the proton temperature anisotropy in this space a strong pattern emerges, with high perpendicular temperature where left-handed parallel modes and proton beams exist, and high parallel temperature where the right-handed perpendicular modes exist. This important result shows that cyclotron and landau damping play important roles in heating the solar wind, but also throws out a big problem. The polarisation of the wave measured is critically dependent on the sampling direction of the spacecraft (radial) and so it seems that the modes present in the radial direction have a disproportionately large effect on proton temperature. I will discuss a few ideas for why this might be true.
• Magnetic Reconnection and Turbulence in Stellar-Convection-Zone-Relevant Laboratory Plasmas
Dr. Jack Hare, Imperial College London, abstract, slides
[#s1113, 17 Jul 2020]
Magnetic reconnection and magnetised turbulence are ubiquitous phenomena in our magnetised universe. These processes have been carefully studied in the photosphere of the sun, in the solar wind, and in laboratory experiments which can recreate these collisionless or weakly collisional conditions. However, these phenomena are also important strongly collisional plasmas, in which the mean free path is shorter than the ion and electron skin depths. One example is the convection zone of the sun, the opaque region beneath the photosphere which is difficult to study through observations. Ryutov noted that this regime is also present in dense z-pinches (Ryutov, 2015), which combine intense magnetic fields with high temperatures and densities. In this talk, I will discuss experiments which use mega-ampere currents to ablate, accelerate and sculpt plasma from initially solid-density targets, creating geometries such as a quasi-two-dimensional reconnection layer in which plasmoids form, or a column of turbulent plasma confined at the axis of an imploding wire-array z-pinch. I will describe new diagnostics for studying the spectrum of turbulent fluctuations in the density, velocity, temperature and magnetic field, and I will present a new pulsed-power facility for studying magnetised high-energy-density plasmas which will be built at MIT. [Ryutov, 2015]: "Characterizing the Plasmas of Dense Z-Pinches." IEEE TPS
• Magnetic pumping model for energizing superthermal particles applied to observations of the Earth's bow shock
Dr. Emily Lichko, U. Arizona, seminar, abstract, slides
[#s1112, 10 Jul 2020]
Energetic particle generation is an important component of a variety of astrophysical systems, from seed particle generation in shocks to the heating of the solar wind. It has been shown that magnetic pumping is an efficient mechanism for heating thermal particles, using the largest-scale magnetic fluctuations. Here we show that when magnetic pumping is extended to a spatially-varying magnetic flux tube, magnetic trapping of superthermal particles renders pumping an effective energization method for particles moving faster than the speed of the waves and naturally generates power-law distributions. We validated the theory by spacecraft observations of the strong, compressional magnetic fluctuations near the Earth’s bow shock from the Magnetospheric Multiscale mission. Given the ubiquity of magnetic fluctuations in different astrophysical systems, this mechanism has the potential to be transformative to our understanding of how the most energetic particles in the universe are generated.
• Testing the Physics of Solar and Stellar Flares with NASA’s Solar Dynamics Observatory and Radiative MHD Simulations
Dr. Mark Cheung, Lockheed Martin, abstract, slides
[#s1088, 11 Nov 2019]
Solar and stellar flares are the most intense emitters of X-rays and extreme ultraviolet radiation in planetary systems. On the Sun, strong flares are usually found in newly emerging sunspot regions. The emergence of these magnetic sunspot groups leads to the accumulation of magnetic energy in the corona. Following magnetic reconnection, the energy released powers coronal mass ejections and heats plasma to temperatures beyond tens of millions of Kelvins. In part one of this talk, we show how extreme UV images of the solar corona taken by NASA’s Solar Dynamics Observatory can be used to quantify the thermal structure and evolution of magnetically active regions on the Sun. The thermal structures inferred from extreme UV observations are consistent with their soft X-ray counterparts. Lessons learned from such studies guide the development of models of flares and eruptions. In the second part of this talk, we present radiative MHD simulations of flares and eruptions with sufficient realism for the synthesis of remote sensing measurements at visible, UV and X-ray wavelengths. These models allow us to explain a number of well-known observational features, including the time profile of the X-ray flux, chromospheric evaporation and condensation, the sweeping of flare ribbons in the lower atmosphere, global coronal waves, and the non-thermal spectral shape of coronal X-ray sources. Implications for how we interpret X-ray spectra from other astrophysical sources will be discussed.
• Plasma astrophysics of neutron stars and black holes
Dr. Sasha Philippov, Center for Computational Astrophysics, slides
[#s1070, 13 Sep 2019]
• Nonthermal particle energization in relativistic plasma turbulence
Dr. Vladimir Zhdankin, Princeton University, abstract, slides
[#s982, 02 May 2019]
I will describe recent numerical progress on understanding turbulence in relativistic collisionless plasmas, as found in high-energy astrophysical systems such as pulsar wind nebulae, black-hole accretion flows, and jets. I will present results from first-principles particle-in-cell simulations of driven turbulence. One main outcome is the confirmation that turbulence can be an efficient and viable astrophysical particle accelerator, producing nonthermal energy distributions with extended power laws, supporting decades-old theoretical ideas. I will also discuss intriguing results on electron-ion energy partition, showing that the dissipation of turbulence naturally produces a two-temperature plasma (with ions much hotter than electrons, as required by models of radiatively inefficient accretion flows). Finally, I will describe recent results on turbulence with strong radiative cooling through inverse Compton scattering, which allows a rigorous statistical steady state to be maintained. I will show that radiative cooling thermalizes the particle distribution and allows intermittent beaming of particles, possibly explaining rapid flares in various astrophysical systems.
• Magnetic turbulence in a plasma wind tunnel at the Bryn Mawr Plasma Laboratory
Prof. David Schaffner, Bryn Mawr College, abstract, slides
[#s984, 25 Apr 2019]
A newly commissioned device at the Bryn Mawr Plasma Laboratory (BMPL) is the first experiment specifically designed to be a magnetically turbulent plasma wind tunnel. Called the Bryn Mawr Magnetohydrodynamic Experiment (BMX), the experiment consists of a plasma gun generated magnetized plasma that is launched down a flux conserving chamber. A high density magnetic pickup probe array and high bit-depth data acquisition system allows for a through exploration of spatial and temporal magnetic fluctuations. This talk presents the first results from the experiment including time and spatial correlation features, magnetic turbulent spectra, and bulk velocity. Plans for upcoming experiments and goals will be discussed.
• Electron energy partition across interplanetary shocks near 1 AU
Dr. Lynn Wilson, NASA Goddard, abstract, slides
[#s983, 18 Apr 2019]
Analysis of 15,314 electron velocity distribution functions (VDFs) within ±2 hours of 52 interplanetary (IP) shocks observed by the Wind spacecraft near 1 AU are presented. The electron VDFs are fit to the sum of three model functions for the cold dense core, hot tenuous halo, and field-aligned beam/strahl component. The halo and beam/strahl are always modeled as bi-kappa VDFs but the core is found to be best modeled by a bi-self-similar, not bi-Maxwellian, for nearly all cases and a bi-kappa for a small fraction of the events. The self-similar distribution deviation from a Maxwellian is a measure of inelasticity in particle scattering from waves and/or turbulence. The range of values defined by the lower and upper quartiles for the kappa exponents are k_ec ~ 5.40--10.2 for the core, k_eh ~ 3.58--5.34 for the halo, and k_eb ~ 3.40--5.16 for the beam/strahl. The lower-to-upper quartile range of symmetric bi-self-similar core exponents are s_ec ~ 2.00--2.04, and asymmetric bi-self-similar core exponents are p_ec ~ 2.20--4.00 for the parallel exponent, and q_ec ~ 2.00--2.46 for the perpendicular exponent. The rest of the parameters will be summarized as well during the talk.
• The interplay of plasma turbulence and magnetic reconnection in producing nonthermal particles
Dr. Luca Comisso, Columbia University, abstract, slides
[#s991, 12 Apr 2019]
Due to its ubiquitous presence, turbulence is often invoked to explain the origin of nonthermal particles in astrophysical sources of high-energy emission. With particle-in-cell simulations, we study decaying turbulence in magnetically-dominated (or equivalently, “relativistic”) pair plasmas. We find that the generation of a power-law particle energy spectrum is a generic by-product of magnetically-dominated turbulence. The power-law slope is harder for higher magnetizations and stronger turbulence levels. In large systems, the slope attains an asymptotic, system-size-independent value, while the high-energy spectral cutoff increases linearly with system size; both the slope and the cutoff do not depend on the dimensionality of our domain. By following a large sample of particles, we show that particle injection happens at reconnecting current sheets; the injected particles are then further accelerated by stochastic interactions with turbulent fluctuations. Our results have important implications for the origin of non-thermal particles in high-energy astrophysical sources.
• Instabilities and Plasma Heating in the Inner Heliosphere: Thermodynamics far from Equilibrium
Prof. Kris Klein, University of Arizona, abstract, slides
[#s981, 28 Mar 2019]
One key feature of the solar wind, a diffuse and high-temperature plasma, is that generally the Coulomb collision frequency is low compared to other dynamic timescales, enabling the plasma to maintain significant deviations from local thermodynamic equilibrium. These departures from LTE, characterized for instance by temperature anisotropies as well as temperature disequilibrium and relative drifts between components, can drive unstable wave growth. In this talk, we discuss recent results that use observations of non-equilibrium distributions at 1 au to determine how frequently unstable waves are driven. Using an automated implementation of Nyquist's instability criterion, we find that half of the intervals from a statistical set of ion velocity distributions support linear instabilities, a much larger fraction than previous estimates. Departures from LTE can also serve as signatures of processes that occurred at an earlier time, before the solar wind was advected to the point of measurement. Using a model of Coulomb relaxation and solar wind expansion, coupled with decades of observations of Hydrogen and Helium temperatures at 1 au, we are able to identify a region within tens of Solar radii of the Sun where strong preferential heating of minor ions is active, producing the observed temperature disequilibrium. The existence and characteristics of this predicted region will be tested by Parker Solar Probe, which will provide in situ plasma and electromagnetic field measurements within 10 Solar radii from the Sun, closer than any previous mission.
• Turbulent "heating" in kinetic plasmas
Dr. Tulasi Parashar, University of Delaware *CANCELLED*, abstract
[#s980, 14 Mar 2019]
Many naturally occurring plasmas are weakly collisional. Examples include Solar Wind, planetary magnetospheres, black hole accretion disks, and intracluster medium. Most of these systems are either observed or believed to be in a turbulent state. Nonlinear interactions cascade fluctuations to kinetic scales where energy is converted from turbulent fluctuations to internal energy. The kinetic nature of these systems makes traditional viscous closure inapplicable. We discuss possible route to increasing the internal energy in kinetic plasma turbulence. Average energy equations for the Vlasov-Maxwell system provide valuable insights into how a collisionless generalization of viscosity is responsible for this conversion into internal energy. Evidence from kinetic simulations as well as multi-spacecraft observations is presented.
• Large-scale solar eruptions and induced small-scale magnetic reconnection
Prof. Xin Cheng, Nanjing University , abstract, slides
[#s979, 08 Mar 2019]
Coming Coronal mass ejections (CMEs) and solar flares are the large-scale and most energetic eruptive phenomena in our solar system and able to release a large quantity of plasma and magnetic flux into the solar wind. When these high-speed magnetized plasmas along with the energetic particles arrive at the Earth, they may interact with the magnetosphere and ionosphere, and seriously affect the safety of human high-tech activities in outer space. To predict CMEs/flares caused space weather effects, we need to elucidate some fundamental but still puzzled questions including in particular the origin and early evolution of CMEs/flares. Theoretically, magnetic flux rope is defined as a coherent magnetic structure with all magnetic field lines wrapping around its central axis. It is believed to be the fundamental structure of CMEs/flares, however, its existence has been lack of direct evidence. In my talk, I will present recent observations, in which the flux rope is found to appear as a coherent plasma channel with a temperature up to 10 million degree. It even pre-exists prior to the eruption. I then show the evolution of the hot channel toward CMEs/flares. Finally, I plan to talk about the properties of magnetic reconnection that takes place in the stretched long current sheet in the wake of the erupting CMEs. Some interesting features including significant heating and nonthermal velocity within the current sheet, intermittent outflows at two ends of the current sheet, and large length-to-width ratio suggest that magnetic reconnection during CMEs/flares may proceed in fragmented and turbulent way.
• Probing Magnetic Reconnection in Solar Flares with Radio Spectral Imaging
Prof. Bin Chen, New Jersey Institute of Technology , abstract
[#s978, 01 Mar 2019]
Flares on the Sun, thanks to their proximity, serve as an outstanding laboratory to test our understanding on magnetic reconnection and the associated magnetic energy release and particle acceleration processes. Flare-accelerated nonthermal electrons in the low solar corona emit radio waves in decimeter-centimeter wavelengths. Observations of these radio waves provide excellent means for tracing the accelerated electrons, and in turn, for probing a variety of physical processes and plasma properties in and around the magnetic reconnection site. The newly available radio spectral imaging capability from recently commissioned telescope arrays opens up a new window for such investigations. I will discuss our recent results of this kind based on observations from the Karl G. Jansky Very Large Array and NJIT’s Expanded Owens Valley Solar Array.
• Magnetic Reconnection Drivers of Solar Eruptions
Dr. Joel Dahlin, NASA Goddard, abstract, slides
[#s977, 14 Feb 2019]
Eruptive solar activity such as coronal mass ejections, eruptive flares, and coronal jets are understood to be powered by highly stressed magnetic fields in the solar corona. It is generally agreed that a key role is played by magnetic reconnection, a fundamental plasma process that drives explosive magnetic energy release via large-scale topological reconfiguration. We report on 3D MHD simulations that definitively demonstrate three distinct roles of magnetic reconnection in the genesis of a coronal mass ejection. The system is initialized with a simple, current-free null point configuration, and energy and structure are injected via small-scale boundary flows. The evolution proceeds as follows: (1) A reconnection-mediated inverse helicity cascade rapidly reconfigures the magnetic fields to form a circular, highly sheared magnetic arcade. (2) The resulting magnetic pressure deforms the coronal null into a horizontal current sheet that reconnects and destabilizes quasi-static force balance by removing restraining tension. (3) The configuration expands, stretching magnetic fields to form a vertical current sheet that reconnects to expel the accumulated shear and drive rapid energy release. We discuss observational signatures of these three forms of reconnection and discuss implications for particle acceleration and solar eruption prediction.
• Mercury‘s Dynamic Magnetosphere
Prof. James Slavin, University of Michigan, abstract, slides
[#s743, 06 Dec 2018]
MESSENGER’s exploration of Mercury has led to many important discoveries and a global perspective on its magnetosphere, exosphere, and interior as a coupled system. Mercury’s proximity to the Sun, weak planetary magnetic field, electrically conducting core, and sodium-dominated exosphere give rise to a highly dynamic magnetosphere unlike that of any other planet. The strong interplanetary magnetic fields so close to the Sun result in a high rate of energy transfer from the solar wind into Mercury’s magnetosphere. Surprisingly, direct solar wind impact on the surface during coronal mass ejection impact has been found to be infrequent. Electric currents induced in Mercury’s highly conducting interior buttress the weak planetary magnetic field against direct impact for all but the strongest solar events. Kinetic effects associated with the large orbits of planetary ions about the magnetic field and the small dimensions of the magnetosphere are observed to significantly affect some fluid instabilities such as Kelvin-Helmholtz waves along the magnetopause. As at Earth, magnetic reconnection, dipolarization fronts, and plasmoid ejection are closely associated with substorms in Mercury’s magnetosphere, and MESSENGER frequently observed energetic electrons with energies of tens to several hundred thousand electron volts. However, no “Van Allen” radiation belts with durable trapping are present.
• The lunar plasma wake and electron phase-space holes
Prof. Ian Hutchinson, MIT, abstract, slides
[#s926, 30 Nov 2018]
Wakes of plasma flowing past unmagnetized bodies like probes, moons, or large particles are usually unsteady. Detailed theory and simulations show instabilities excited by the velocity distribution distortions give rise to electron holes (soliton-like BGK modes). We have recently discovered from spacecraft observations that the solar wind wake of the moon is full of electron holes, in agreement with predictions. Transverse instability of these holes determines their evolution and persistence and how they eventually merge into the background plasma.

2 related papers:
"Prediction and Observation of Electron Instabilities and Phase Space Holes Concentrated in the Lunar Plasma Wake", Ian H. Hutchinson, David M. Malaspina, Geophysical Res. Lett. 2018

"Transverse instability of electron phase-space holes in multi-dimensional Maxwellian plasmas", I. H. Hutchinson J. Plasma Phys. 2018
• On the role of magnetic reconnection in kinetic-range turbulence and the existence of cascades in the entire phase space from hybrid-Vlasov-Maxwell simulations
Silvio Cerri, Princeton University , abstract, slides
[#s735, 30 May 2018]
Understanding the properties of turbulent fluctuations and how turbulent energy is dissipated in weakly collisional plasmas is a fundamental step towards understanding how turbulence feeds back on the evolution of several astrophysical systems. In this context, space plasmas are probably the best laboratory for the study of plasma turbulence in a weakly collisional regime, as the Earth’s environment has become accessible to increasingly accurate direct measurements. In situ observations of the solar wind and the terrestrial magnetosheath have indeed provided relevant constraints on the turbulent energy spectra, determining the typical values of their slopes and revealing the presence of breaks in the electromagnetic fluctuation cascade at kinetic scales. A first break in the turbulent spectrum is indeed encountered at the proton kinetic scales and separates the so-called “MHD inertial range” spectrum from the kinetic spectrum that arises at scales smaller than the proton gyroradius (also referred to as the “dissipation” or “dispersion” range). Such transition is a clear evidence of a change in the physics underlying the cascade process, and its understanding is today a matter of a strong debate. Very high resolution measurements by MMS have also recently pointed out the presence of structures in the particle (electron) distribution function that can be interpreted as a cascade in velocity space.
In this talk I will present some recent developments in the investigation of the properties of kinetic-range turbulence via high-resolution hybrid-kinetic (fully-kinetic ions and fluid electrons) simulations both in 2D and 3D. In particular, I will show the first numerical evidence that has led to the suggestion of a link between magnetic reconnection, ion break and turbulent energy transfer in the sub-ion-gyroradius cascade[1,2] (also known as “reconnection-mediated scenario” for plasma turbulence). Finally, I will show the first evidence for a six-dimensional (“dual”) phase-space cascade of ion-entropy fluctuations in a 3D3V simulation of electromagnetic turbulence: such phase-space cascade is shown to be anisotropic with respect to the background magnetic fleld in both real and velocity space and suggests that both linear and non-linear phase mixing are simultanously at work[3].
[1] S. S. Cerri & F. Califano, New J. Phys. 19, 025007 (2017)
[2] Luca Franci, Silvio Sergio Cerri et al., Astrophys. J. Lett. 850, L16 (2017)
[3] S. S. Cerri, M. W. Kunz & F. Califano, Astrophys. J. Lett. 856, L13 (2018)
• Magnetic Reconnection during Turbulence and the Role it Plays in Dissipation and Heating
Mike Shay, U. Delaware , abstract, slides
[#s707, 09 May 2018]
Turbulence plays an important role in many plasmas, including those in accretion disks, in the heliosphere, and in the laboratory. In plasmas with low collisionality, such as those in the heliosphere, exactly how this turbulent energy damps away is an open question, with ramifications for the heating of the solar corona and the solar wind. Magnetic reconnection, where magnetic field lines break and reform in a plasma, is one possible mechanism for damping this turbulent energy and heating the plasma, but the role it may play is uncertain. Recently, however, significant progress has been made in understanding plasma heating in isolated reconnection sites. Can this new knowledge shed light on the properties of plasma heating during turbulence?
In this talk, after reviewing our understanding of heating due to reconnection, I will lay out a framework for applying reconnection heating predictions to turbulent systems, and show initial results for testing this framework using fully kinetic PIC simulations. In addition, I will discuss recent MMS observations of reconnection in Earth's turbulent magnetosheath. I will then explore the statistics of magnetic reconnection in kinetic simulations of turbulence. By statistics, I mean the number of x-lines, the spread of reconnection rates, and how these quantities vary in time. How these statistics vary in different turbulence regimes and its impact on reconnection heating will be discussed.
• Collisionless damping of slow magnetosonic waves (and related compressional fluctuations)
Bill Dorland, University of Maryland , abstract
[#s663, 30 Mar 2018]
Compressional perturbations are observed in the solar wind even when the collision time is much longer than an inferred wave period. This is puzzling. Lithwick & Goldreich argued that the parallel wavenumbers of the slow modes would be inherited from the Alfvén cascade, which would itself be well-described as being in critical balance. For most parameters, this argument favors rapid damping of compressional fluctuations, $\gamma \sim k_\parallel v_A \sim k_\perp v_\perp$. Schekochihin et al. argued instead that the compressional perturbations would evolve in Lagrangian fashion, maintaining their original (possibly very long) wavelengths along the magnetic field, even as the field itself developed ever-shorter parallel wavelengths. Although compressional waves would still experience Landau and/or Barnes damping in this picture, the rate could be very small. Kanekar et al. observed that stochastic echoes could “fluidize” the compressional fluctuations, allowing them to evade collisionless damping altogether. It remains unclear which mechanism is dominant, if any. I will present recent work on this problem by R. Meyrand, A. Kanekar, A. Schekochihin, and myself.
• Magnetic Reconnection in MHD and Kinetic Turbulence
Nuno Loureiro, MIT , abstract
[#s631, 21 Feb 2018]
Recent works have revisited the current understanding of Alfvénic turbulence to account for the role of magnetic reconnection [1-3]. Theoretical arguments suggest that reconnection inevitably becomes important in the inertial range, at the scale where it becomes faster than the eddy turn over time. This leads to a transition to a new sub-inertial interval, suggesting a route to energy dissipation that is fundamentally different from that envisioned in the usual Kolmogorov-like phenomenology.
These concepts can be extended to weakly collisional plasmas, where reconnection is enabled by electron inertia rather than resistivity [4,5]. Although several different cases must then be considered (whether the eddies themselves are on MHD or kinetic scales, whether the plasma beta is large or small, etc.), a common result to all of them is that the energy spectrum exhibits a scaling with the perpendicular wave number that scales between $k_\perp^{−8/3}$ and $k_\perp^{−3}$, in favourable agreement with many numerical results and observations.
This talk aims to review these results, and discuss their implications.
[1] Nuno F. Loureiro & Stanislav Boldyrev, Phys. Rev. Lett. 118, 245101 (2017)
[2] A. Mallet, A. A. Schekochihin & B.D.G. Chandran, Mon. Not. R. Astron. Soc. 468, 4862 (2017)
[3] Stanislav Boldyrev & Nuno F. Loureiro, Astrophys. J. 844, 125 (2017)
[4] Nuno F. Loureiro & Stanislav Boldyrev, Astrophys. J. 850, 182 (2017)
[5] Alexander A. Schekochihin & Benjamin D. G. Chandran, J. Plasma Phys. 83, 905830609 (2017)