"Particle-in-cell Simulations of the Scattering of Energetic Electrons by Whistler Waves"

by Hanqing Ma

Friday, October 28, 2022 -- 12:00 p.m.
Large Conference Room, 1207 Energy Research Facility

Advisors:  Professor James Drake and Dr. Marc Swisdak

The transit time of relativistic electrons out of energy-release regions is of the order of 0.01 s, which exceeds the observed decay time of hard-X ray emission by two orders of magnitude. Theory shows that the primary energy gain during acceleration is in the parallel direction, but observation of gyrosynchrotron emission shows significant energy in the perpendicular direction. In summary, there is significant evidence pointing to the existence of a mechanism that inhibits electron transport and scatters the velocity. We conduct particle-in-cell simulations to investigate the inhibition of energetic electrons’ motion by whistler waves. The initial electron velocity distribution function is the combination of a bi-kappa distribution with large temperature anisotropy and a drifting Maxwellian distribution, which represent the heat flux and the return current, respectively. We adopt recycling boundaries to preserve the magnetic field fluctuations during the simulation: electrons hitting the boundaries are re-injected at the boundaries with a kappa flux or drifting Maxwellian distribution. The initial distribution excites an oblique whistler wave through the fan instability, and later the oblique whistler wave is excited by the heat flux injection at the bottom boundary. In the end, the system is stable and magnetic field fluctuations are maintained in the box. Power spectra show a combination of oblique and perpendicular modes arising from oblique whistler waves and the filamentary structure of the Weibel instability, respectively. Velocity distributions show that the whistler wave inhibits the heat flux and makes the total distribution isotropic. Electron trajectories show a circular diffusion path that agrees with theory. The simulation results could provide insights into the mechanisms inhibiting energetic particle transport in the corona.

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