Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session TI01: Reconnection and Space PlasmasLive Streamed
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Chair: Luca Comisso, Columbia Univ. Room: Ballroom 100 A |
Thursday, October 20, 2022 9:30AM - 10:00AM |
TI01.00001: Magnetic reconnection at multi-scales from the terrestrial bow shock to the magnetopause Invited Speaker: Jonathan Ng The quasi-parallel bow shock creates highly turbulent conditions favourable for magnetic reconnection in the shock region, its downstream magnetosheath, and at the magnetopause. Upstream of the bow shock, the presence of reflected particles excites ultra-low frequency waves, which interact nonlinearly as they are convected towards the bow shock, creating plasma structures which can affect the magnetosheath and magnetopause. At the bow shock, MMS observations have shown that both regular and electron-scale reconnection occur, while other observations show that high-speed jets – regions of enhanced dynamic pressure in the magnetosheath – can trigger reconnection at the magnetopause. We discuss 3D global hybrid and local fully kinetic simulations at and downstream of the quasi-parallel shock. In the hybrid simulations, we show how the effects of foreshock turbulence and the impact of high-speed jets on the magnetopause can lead to magnetopause reconnection events in addition to the quasi-steady reconnection that takes place due to southward magnetic fields, and discuss how this compares to observations. In the 3D kinetic simulations, we study the evolution of current sheets in the shock transition region, showing different configurations not captured by 2D simulations. The electron-scale reconnection is fast and transient, and we perform comparisons to MMS observations. |
Thursday, October 20, 2022 10:00AM - 10:30AM |
TI01.00002: Are cosmic voids filled with reconnecting magnetic fields from the early Universe? Invited Speaker: David N Hosking It has been suggested that the weak magnetic field hosted by the intergalactic medium (IGM) in cosmic voids might be a relic from the early Universe. If so, the strength and coherence length of void fields could be "predicted" from reasonable assumptions about the properties of the primordial field at its genesis, provided the evolution of the field in the intervening time were understood. Inversely, cosmological models of primordial magnetogenesis could be constrained by precise observations of the void fields. In this talk, I shall argue that the plasma physics of magnetic reconnection in the primordial plasma is key to understanding the evolution of the primordial fields; the reconnection-controlled decay theory that I present resolves a number of historical discrepancies between theory, numerics, and observations. |
Thursday, October 20, 2022 10:30AM - 11:00AM |
TI01.00003: First-Principles Theory of the Rate of Magnetic Reconnection Invited Speaker: Yi-Hsin Liu The rate of magnetic reconnection is of the utmost importance in a variety of processes because it controls, for example, the rate energy is released in solar flares, the speed of the Dungey convection cycle in Earth's magnetosphere, and the energy release rate in harmful geomagnetic substorms. It is known from numerical simulations and satellite observations that the rate is approximately 0.1 in normalized units, but despite years of effort, a full theoretical prediction has not been obtained. Here, we present a first-principles theory for the reconnection rate in electron-ion collisionless plasmas, and show that the same prediction explains why Sweet-Parker reconnection is considerably slower. The key consideration of this analysis is the pressure at the reconnection site (i.e., the x-line). We show that the Hall electromagnetic fields in antiparallel reconnection cause an energy void, equivalently a pressure depletion, at the x-line, so the reconnection exhaust opens out, enabling the fast rate of 0.1. If the energy can reach the x-line to replenish the pressure, the exhaust does not open out. In addition to heliospheric applications, these results are expected to impact reconnection studies in planetary magnetospheres, magnetically confined fusion devices, and astrophysical plasmas. |
Thursday, October 20, 2022 11:00AM - 11:30AM |
TI01.00004: Efficient Nonthermal Ion and Electron Acceleration in 3D Magnetic Reconnection Invited Speaker: Qile Zhang Solar flare and Earth’s magnetotail observations show simultaneous acceleration of ions and electrons into power-law energy distributions extending to high energy. This suggests a common magnetic-reconnection acceleration process but the underlying physics is not well understood. During magnetic reconnection, energetic particles undergo a universal Fermi acceleration process involving the curvature drift of particles. However, the efficiency of this mechanism is limited by the trapping of energetic particles within flux ropes. Using 3D fully kinetic simulations, we demonstrate that the flux-rope kink instability leads to field-line chaos in weak-guide-field regimes where the Fermi mechanism is most efficient, thus allowing particles to transport out of flux ropes and undergo further acceleration. As a consequence, both ions and electrons form clear power laws which contain a significant fraction of the released energy. The low-energy bounds, which control the nonthermal energy contents, are determined by the injection physics, while the high-energy cutoffs are limited only by the system size. In contrast, in the higher-guide-field regimes, field-line chaos and efficient acceleration comes from 3D overlapping oblique tearing flux ropes of large sizes. As a result, both species also form power laws consistent with Fermi acceleration, with indices softer than weak-guide-field regimes. Interestingly, these oblique flux ropes are also subject to kink instability. These basic results have strong relevance to observations of nonthermal particle acceleration in both the solar corona and magnetotail. |
Thursday, October 20, 2022 11:30AM - 12:00PM |
TI01.00005: Velocity-Space Structure of Terms in the Electron Vlasov Equation: MMS Magnetopause Observations and Model Results Invited Speaker: Jason Shuster The Vlasov equation describes collisionless plasmas in the continuum limit and applies to fundamental plasma energization phenomena occurring in Earth's magnetosphere, throughout the heliosphere, and in laboratory fusion experiments. Because this equation governs the evolution of plasmas in six-dimensional phase space, studies of its structure typically rely on numerical or analytical approaches. In this work, each term of the Vlasov equation is determined from direct observations of electron phase-space density gradients measured by the Magnetospheric Multiscale (MMS) spacecraft in the vicinity of magnetic reconnection occurring at Earth's magnetopause. The unprecedented temporal, spatial, and velocity-space resolution offered by the MMS tetrahedron enables us to identify the electron velocity-space distribution that supports the pressure divergence within electron-scale current layers. We characterize the relationship between the distribution's velocity-space structure and spatial gradients in the bulk plasma moments: unipolar, bipolar, and ring structures in the electron phase-space density gradient terms are compared to a simplified Maxwellian model and correspond to gradients in density, velocity, and temperature, respectively. We compare the MMS observations to exact Vlasov-Maxwell solutions and particle-in-cell simulations of asymmetric reconnection suitable for modeling Earth's magnetopause. Our results provide a perspective relevant to how the electron pressure divergence develops to violate the frozen-in condition and sustain electron-scale energy conversion processes, such as the reconnection electric field, in collisionless plasmas. This work is immediately relevant to the study of fundamental energy conversion processes, including electron diffusion regions fueling magnetic reconnection, kinetic-scale turbulence, and wave-particle interactions consistent with Landau damping that were recently reported using MMS data from Earth's turbulent magnetosheath. |
Thursday, October 20, 2022 12:00PM - 12:30PM |
TI01.00006: Lev D. Landau and Lyman Spitzer Jr. Award for Outstanding Contributions to Plasma Physics: Unraveling the Physics of Particle Energization in Space and Astrophysical Plasmas: The Field-Particle Correlation Technique Invited Speaker: Gregory G Howes Under the weakly collisional conditions typical of space and astrophysical plasmas, fundamental plasma processes such as kinetic plasma turbulence, collisionless magnetic reconnection, collisionless shocks, and kinetic instabilities govern the transport of energy across scales and the consequent energization of the plasma, through either the heating of the plasma species or the acceleration of a small fraction of particles to high energy. Many of these energization mechanisms remain poorly understood, but kinetic simulations and spacecraft observations present valuable opportunities to improve our understanding of the fundamental kinetic physics. The field-particle correlation technique was devised to combine measurements of the particle velocity distribution functions and electromagnetic fields at a single point in space to generate a characteristic velocity-space signature that can be used to identify the kinetic mechanism of particle energization and quantify the energization rate. In this talk, the kinetic plasma theory underlying the field-particle correlation technique will be reviewed. The technique has been successfully applied to identify different physical mechanisms involved in the dissipation of kinetic plasma turbulence, the acceleration of particles at collisionless shocks, and the heating of electrons in collisionless magnetic reconnection. The unique velocity-space signatures of these different processes can be compiled to generate a "Rosetta stone" for the definitive identification of different particle energization mechanisms in space and astrophysical plasmas. Application of the technique to large statistical samples of spacecraft data holds the promise to create predictive models of energy transport and plasma heating in turbulence, reconnection, shocks, and instabilities as a function of the plasma and system parameters |
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