Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsh52a..03g&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #SH52A-03
Physics
2114 Energetic Particles (7514), 7807 Charged Particle Motion And Acceleration, 7833 Mathematical And Numerical Techniques (0500, 3200), 7859 Transport Processes
Scientific paper
We study the acceleration of suprathermal electrons by planar collisionless shocks or compressions moving in a plasma containing a turbulent magnetic field. Here we focus on the case where the disturbance propagates normal to the mean magnetic field. For electrons whose energy is less than a few tens of keV, the shock appears as a smooth compression of the plasma and fields. Moreover, the irregular magnetic field is dominated by ion spatial scales that are far greater than the gyroradius of the electrons. Since there are no scales of the order of the suprathermal electron gyroradius, the first adiabatic invariant is conserved. Because the field lines meander randomly in space and intersect the disturbance in several locations, the electrons cross it several times, depending on the particular field line. This leads to a net energy gain. The process is much like first-order Fermi acceleration of ions at a parallel shock, except that here, the electrons are adiabatic and cross the shock due to the random meandering of magnetic field lines. The energy gain arises when the electrons change direction as they follow the random field lines and undergo curvature drift in the motional electric field. At low electron energies, the energy gain is entirely in the direction parallel to the local field because of the conservation of the first adiabatic invariant. As the electrons gain significant energy, they begin to resonate with the ambient ion-scale waves, isotropize, and the usual diffusive shock acceleration occurs. We derive the resulting energy spectrum. We also compare the analytic predictions with numerical simulations.
Giacalone Joe
Jokipii Randy J.
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