Bi-directional streaming of particles accelerated at interplanetary shocks: the case-study of a STEREO reverse shock and statistical analysis of shock-spikes from ACE archive

Abstract:

We present a detailed interpretation of the pitch-angle anisotropy and intensity of supra-thermal ions (10-40 keV and 81-92 keV protons) near a fast quasi-perpendicular reverse shock measured by Solar Terrestrial Relations Observatory Ahead (STEREO-A) on 2008 March 9th. The measured intensity profile of the supra-thermal particles exhibits an enhancement, or ''spike'', at the time of the shock arrival and pitch-angle anisotropies before the shock arrival are bi-modal, jointly suggesting the ions are trapped in near-scatter-free regime along magnetic field lines which intersect the shock at both ends. We run test-particle simulations with pre-existing upstream magnetostatic fluctuations advected across the shock. Our model results are consistent with the measured bi-modal upstream anisotropy, the nearly field-aligned anisotropies far upstream the shock arrival (16 minutes), as well as the ''pancake-like'' anisotropies downstream of the shock (10 minutes). These results, in agreement with earlier works, suggest a dominant role of the large-scale structure of the magnetic field in forging the early-on phase of particle acceleration at shocks. In addition, we present some preliminary results on a comprehensive statistical analysis on shock spikes from ACE/STEREO archives.