PPPL

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
  • 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
  • provide a forum to facilitate cross-fertilization between laboratory plasma physics, astrophysics, and heliospheric science
Heliophysics seminars are usually held Thursdays, @2 PM, in the Theory Conference Room, T169.

Past

  • Plasma astrophysics of neutron stars and black holes
    Dr. Sasha Philippov, Center for Computational Astrophysics
    [#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)