Electron acceleration at nearly perpendicular collisionless shocks 3. Downstream distributions
Abstract
Spacecraft observations at the Earth's bow shock and at interplanetary shocks have established that the largest fluxes of accelerated suprathermal electrons occur in socalled shock spike events immediately downstream of the shock ramp. Previous theoretical efforts have mainly focused on explaining upstream energetic electron beams. Here we investigate the general motion and acceleration of energetic electrons in a curved, nearly perpendicular shock by numerically integrating the orbits of solar wind halo electrons in shock fields generated by a hybrid simulation (core electron fluid and kinetic ions). Close to the angle θ_{Bn}=90° between the upstream magnetic field and shock normal, the calculations result in a (perpendicular) temperature increase proportional to the magnetic field ratio and give the highest phase space densities in the overshoot. For a steep distribution, the temperature change can correspond to an enhancement of the distribution by several orders of magnitude. These results are in agreement with predictions from adiabatic mapping. With smaller angles θ_{Bn}, the overshoot and downstream densities fall off quickly, because the adiabatic energy gain is less and fewer electrons transmit. The shock curvature also leads to an accumulation of electrons close to 90°. Without pitch angle scattering, energization is only significant within a few (~5° to 10°) degrees of the point of tangency. However, shock spike events appear to be observed more easily and farther away from 90°.
Given that over a region of several degrees around 90° the theory gives enhancements of up to ~4 orders of magnitude, such electrons could in principle account for the typically observed enhancements of 1 to 2 orders of magnitude, if they were distributed over θ_{Bn}. To test the idea that scattering could efficiently redistribute the energetic electrons, we have conducted test particle simulations in which artificial pitch angle scattering is included. We find that such a process can indeed be very efficient and can explain observations of shock spike events far away from θ_{Bn}~90°. The scattering naturally leads to much higher phase space densities at smaller θ_{Bn} than what a local onedimensional mapping would predict and thus can account for an observed discrepancy with adiabatic theory stated in the literature.
 Publication:

Journal of Geophysical Research
 Pub Date:
 February 1994
 DOI:
 10.1029/93JA01643
 Bibcode:
 1994JGR....99.2537K
 Keywords:

 Bow Waves;
 Collisionless Plasmas;
 Electron Acceleration;
 Shock Waves;
 Solar Wind;
 High Energy Electrons;
 Interplanetary Magnetic Fields;
 Plasma Drift;
 Shock Wave Interaction;
 Astrophysics;
 Interplanetary Physics: Interplanetary shocks;
 Space Plasma Physics: Charged particle motion and acceleration;
 Space Plasma Physics: Numerical simulation studies;
 Space Plasma Physics: Shock waves