SIMULATION OF ELECTRON ACCELERATION AT COLLISIONLESS PLASMA SHOCKS
Robert Edward Lowe
Queen Mary, University of London
2001
Thesis submitted for the degree of
Doctor of Philosophy (PhD)
of the University of London
The acceleration of charged particles to super-thermal energies is widely observed in Astrophysics, where it is a common mechanism for kinetic and magnetic energy release. In solar flares, energetic particles account for a significant fraction of the released energy. We review observations of energetic electrons in solar flares, particularly in the form of X-ray spectra, and discuss the mechanisms and flare models that have been proposed for their acceleration. We argue that direct in situ measurements of the Earth's bow shock can be used to examine the role played by shocks in the acceleration process and thus aid our understanding of solar flare energy release. Simulations of quasi-perpendicular collisionless shocks show ripples in the density and magnetic field moving along the shock ramp. Using a two dimensional hybrid code, we simulate these ripples and determine their speed along the shock, which is consistent with the overshoot Alfvén speed. This suggests a driving instability that is related to surface processes. We also describe a parallel three dimensional hybrid code that we are writing to further investigate the ripple properties. Using our hybrid simulation and a relativistic test particle code, we examine the effect of ripples on electron acceleration. We show how trapping by this two dimensional structure can cause electrons to be convected downstream with the magnetic field, despite having magnetic moments which suggest that they should be reflected upstream. These electrons undergo considerable Fermi acceleration during the shock transition and may explain observations of an energetic population of electrons downstream of Earth's bow shock. We investigate the resulting electron energy spectrum and compare the simulated power law indices with those derived from X-ray spectra and in situ measurement.
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Rob Lowe
Last Revision : 1st March 2003