arXiv subjects

Marc Swisdak

Publications and source records attributed to Marc Swisdak.

24 records · Page 2Linked to original sources

arXiv2021

Electron Acceleration during Macroscale Magnetic Reconnection

The first self-consistent simulations of electron acceleration during magnetic reconnection in a macroscale system are presented. Consistent with solar flare observations the spectra of energetic electrons take the form of power-laws that extend more than two decades in energy. The drive mechanism for these nonthermal electrons is Fermi reflection in growing and merging magnetic flux ropes. A strong guide field is found to suppress the production of nonthermal electrons by weakening the Fermi drive mechanism. For a weak guide field the total energy content of nonthermal electrons dominates that of the hot thermal electrons even though their number density remains small. Our results are benchmarked with the hard x-ray, radio and extreme ultra-violet (EUV) observations of the X8.2-class solar flare on September 10, 2017.

physics.plasm-ph↗

arXiv2019

Large-Scale Parallel Electric Fields and Return Currents in a Global Simulation Model

A new computational model, kglobal, is being developed to explore energetic electron production via magnetic reconnection in macroscale systems. The model is based on the discovery that the production of energetic electrons during reconnection is controlled by Fermi reflection in large-scale magnetic fields and not by parallel electric fields localized in kinetic scale boundary layers. Thus, the model eliminates these boundary layers. However, although the parallel electric fields that develop around the magnetic x-line and associated separatrices are not important in producing energetic electrons, there is a large scale electric field that kickstarts the heating of low-energy electrons and drives the cold-electron return current that accompanies escaping energetic electrons in open systems. This macroscale electric field is produced by magnetic-field-aligned gradients in the electron pressure. We have upgraded kglobal to include this large-scale electric field while maintaining energy conservation. The new model is tested by exploring the dynamics of electron acoustic modes which develop as a consequence of the presence of two electron species: hot kinetic and cold fluid electrons. Remarkably, the damping of electron acoustic modes is accurately captured by kglobal. Additionally, it has been established that kglobal correctly describes the dynamics of the interaction of the parallel electric field with escaping hot electrons through benchmarking simulations with the Particle-In-Cell (PIC) code p3d.

physics.plasm-ph↗

arXiv2019

Decomposition of Plasma Kinetic Entropy into Position and Velocity Space and the Use of Kinetic Entropy in Particle-in-Cell Simulations

We describe a systematic development of kinetic entropy as a diagnostic in fully kinetic particle-in-cell (PIC) simulations and use it to interpret plasma physics processes in heliospheric, planetary, and astrophysical systems. First, we calculate kinetic entropy in two forms -- the ``combinatorial'' form related to the logarithm of the number of microstates per macrostate and the ``continuous'' form related to $f \ln f$, where $f$ is the particle distribution function. We discuss the advantages and disadvantages of each and discuss subtleties about implementing them in PIC codes. Using collisionless PIC simulations that are two-dimensional in position space and three-dimensional in velocity space, we verify the implementation of the kinetic entropy diagnostics and discuss how to optimize numerical parameters to ensure accurate results. We show the total kinetic entropy is conserved to three percent in an optimized simulation of anti-parallel magnetic reconnection. Kinetic entropy can be decomposed into a sum of a position space entropy and a velocity space entropy, and we use this to investigate the nature of kinetic entropy transport during collisionless reconnection. We find the velocity space entropy of both electrons and ions increases in time due to plasma heating during magnetic reconnection, while the position space entropy decreases due to plasma compression. This project uses collisionless simulations, so it cannot address physical dissipation mechanisms; nonetheless, the infrastructure developed here should be useful for studies of collisional or weakly collisional heliospheric, planetary, and astrophysical systems. Beyond reconnection, the diagnostic is expected to be applicable to plasma turbulence and collisionless shocks.

physics.plasm-ph↗

arXiv2018

Characterizing Ion Flows Across a Dipolarization Jet

The structure of dipolarization jets with finite width in the dawn-dusk direction relevant to magnetic reconnection in the Earth's magnetotail is explored with particle-in-cell simulations. We carry out Riemann simulations of the evolution of the jet in the dawn-dusk, north-south plane to investigate the dependence of the jet structure on the jet width in the dawn-dusk direction. We find that the magnetic field and Earth-directed ion flow structure depend on the dawn-dusk width. A reversal in the usual Hall magnetic field near the center of the current sheet on the dusk side of larger jets is observed. For small widths, the maximum velocity of the Earthward flow is significantly reduced below the theoretical limit of the upstream Alfvén speed. However, the ion flow speed approaches this limit once the width exceeds the ion Larmor radius based on the normal magnetic field, $B_z$.

physics.space-ph↗

arXiv2011

The Acceleration of Ions in Solar Flares During Magnetic Reconnection

The acceleration of solar flare ions during magnetic reconnection is explored via particle-in-cell simulations that self-consistently follow the motions of both protons and $α$ particles. We demonstrate that the dominant ion heating during reconnection with a guide field (a magnetic component perpendicular to the reconnection plane) results from pickup behavior during the entry into reconnection exhausts. In contrast with anti-parallel reconnection, the temperature increment is dominantly transverse, rather than parallel, to the local magnetic field. The comparison of protons and alphas reveals a mass-to-charge ($M/Q$) threshold in pickup behavior that favors heating of high $M/Q$ ions over protons, which is consistent with impulsive flare observations.

astro-ph.SR↗

arXiv2009

The Weibel Instability inside the Electron-Positron Harris Sheet

Recent full-particle simulations of electron-positron reconnection have revealed that the Weibel instability plays an active role in controlling the dynamics of the current layer and maintaining fast reconnection. A four-beam model is developed to explore the development of the instability within a narrow current layer characteristic of reconnection. The problem is reduced to two coupled second-order differential equations, whose growing eigenmodes are obtained via both asymptotic approximations and finite difference methods. Full particle simulations confirm the linear theory and help probe the nonlinear development of the instability. The current layer broadening in the reconnection outflow jet is linked to the scattering of high-velocity streaming particles in the Weibel-generated, out-of-plane magnetic field.

physics.space-ph↗