Diffusive Shock Acceleration Of Electrons At An Interplanetary Shock Observed On 21 Feb 1994

Abstract

For the diffusive shock acceleration process to work, we need both upstream and downstream scattering centers. For ions of energies > ∼ several tens of keV, the necessary scattering centers are identified as magneto-hydrodynamic (Alfven) waves, whose frequency determined by the cyclotron resonance condition, (1) becomes ∼ MeV can resonate with and be scattered by Alfven waves of the frequency range similar to the above waves. Electrons of surpathermal energies (< ∼ several tens of keV), on the other hand, can resonate only with waves of much higher frequencies (1–10 Hz), which are no longer on the Alfven wave branch but on the whistler wave branch. Difficulty arises from the matching condition on the helicities of waves: To resonate with electrons streaming away from a shock front, waves should have the right-hand helicity (RHH). Since whistler waves propagating away from the shock purely parallel to the background magnetic field have a right-hand polarization but a left-hand helicity (LHH), we should take into account of obliquely propagating whistler waves which consist of both LHH and RHH. However, since the RHH component has amplitude much less than that of LHH component, the wave-particle cyclotron interaction between upstream going electrons and oblique whistler waves can occur with much reduced efficiency. Because of this reason, Levinson (1992) obtained a high threshold Mach number for a MHD shock to be able to excite upstream whistler waves self-consistently by accelerated electrons; (2) which is prohibitively high for heliospheric shocks (β e is the ratio between the electron thermal pressure and magnetic pressure, which is the order of 0.1-1 for the typical solar wind condition). Therefore, for electron DSA processes to be possible at heliospheric shocks, either the following two conditions should be satisfied: 1. Injection of ∼ MeV electrons by other processes.