Computational Studies of Particle Dynamics during Magnetic Reconnection

Transcription

1 Computational Studies of Particle Dynamics during Magnetic Reconnection Rebecca Melkerson (Washington and Lee University) Marc Swisdak, Ph.D. (UMD); James Drake, Ph.D. (UMD)

2 Background & Motivation During reconnection, magnetic field lines change topology and convert magnetic energy to kinetic and thermal particle energies. 11 August 2017 TREND 2017: Computational Studies of Particle Dynamics during Magnetic Reconnection 2

3 Background & Motivation Reconnection occurs in many space and astrophysical plasmas. The resultant charged particles have potentially hazardous consequences upon reaching earth. image by Soerfm, distributed under CC-BY-SA-3.0 license 11 August 2017 TREND 2017: Computational Studies of Particle Dynamics during Magnetic Reconnection 3

4 About the Simulations Particle-in-cell (PIC) simulations on massively parallel code 2D or 3D: reconnection occurs in xy-plane Code tracks E, B, J &, J ', P &, P ' (pressure tensors), n &, and n ' Example out of plane electron current: Jez 11 August 2017 TREND 2017: Computational Studies of Particle Dynamics during Magnetic Reconnection 4

5 Power Laws in Electron Energy Distributions Power laws arise when the governing physical processes are nominally scale invariant. In-situ observations of magnetic reconnection show power law (non-thermal) electron energy distribution production, but simulations have difficulty producing them. M. Øieroset et. al August 2017 TREND 2017: Computational Studies of Particle Dynamics during Magnetic Reconnection 5

6 Power Laws in Electron Energy Distributions There has been a recent claim of power law production in a 3D simulation, so we looked for power laws in a 3D simulation with similar parameters (guide field of 2 vs 1). Buechner et. al Buechner et. al E averaged over z direction Spatial distribution of E through central plane 11 August 2017 TREND 2017: Computational Studies of Particle Dynamics during Magnetic Reconnection 6

7 Power Laws in Electron Energy Distributions We are not convinced that the data shows a power law. Linear span is only ~1 order of magnitude. Gaussians can add to give illusion of linear region. Solid: initial Dashed: final f(e) Buechner et. al Energy 11 August 2017 TREND 2017: Computational Studies of Particle Dynamics during Magnetic Reconnection 7

8 Effect of β on Electron Energization We run 2D simulations with 5 different values for β, the ratio of thermal to magnetic pressure. We utilize a guiding-center model to evaluate electron energization: -. = E -/ J + p &, + m & n & u 6 &, u 7 κ + : ;,< >= + u = >/ 7 B The terms represent direct electric field acceleration fermi acceleration acceleration by magnetic moment conservation 11 August 2017 TREND 2017: Computational Studies of Particle Dynamics during Magnetic Reconnection 8

9 Effect of β on Electron Energization Energization Magnitude Energization vs Log(β) log(β ) Direct E field Fermi Magnetic moment Direct E field Fermi Magnetic moment ENERGIZATION KEY: Direct E field, increased particles per grid Fermi, increased ppg Magnetic moment, increased ppg Example figures. Graph values obtained by averaging over boxes as shown. 11 August 2017 TREND 2017: Computational Studies of Particle Dynamics during Magnetic Reconnection 9

10 Summary & Acknowledgements We analyzed a 3D simulation with similar parameters to the one claimed to show power laws, yet we still do not believe the data yields non-thermal electronenergydistributions. We find that Fermi acceleration is generally the most significant source of electron energization, with an uptick for low β, but we do not yet understand why. REU program sponsored by the National Science Foundation Award Number: PHY August 2017 TREND 2017: Computational Studies of Particle Dynamics during Magnetic Reconnection 10