# 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.

### Past

• 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
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)