Bi-directional streaming of particles accelerated at the STEREO-A shock on 9th March 2008

TL;DR

This study interprets particle anisotropy and intensity near a shock on 2008 March 9th, showing how large-scale magnetic field structures influence early particle acceleration, supported by simulations matching observations.

Contribution

It provides a detailed interpretation of particle anisotropies and intensities at a shock, emphasizing the role of large-scale magnetic structures, supported by test-particle simulations.

Findings

Bi-modal upstream anisotropy indicates trapping of ions.

Simulations reproduce observed anisotropies and intensity spikes.

Large-scale magnetic field structures dominate early particle acceleration.

Abstract

We present an interpretation of anisotropy and intensity of supra-thermal ions near a fast quasi-perpendicular reverse shock measured by Solar Terrestrial Relations Observatory Ahead (ST-A) on 2008 March 9th. The measured intensity profiles of the supra-thermal particles exhibit an enhancement, or "spike", at the time of the shock arrival and pitch-angle anisotropies before the shock arrival are bi-modal, jointly suggesting trapping of near-scatter-free ions along magnetic field lines that intersect the shock at two locations. We run test-particle simulations with pre-existing upstream magnetostatic fluctuations advected across the shock. The measured bi-modal upstream anisotropy, the nearly field-aligned anisotropies up to ~15 minutes upstream of the shock, as well as the "pancake-like" anisotropies up to ~10 minutes downstream of the shock are well reproduced by the simulations. These results, in agreement with earlier works, suggest a dominant role of the large-scale structure (100s of supra-thermal proton gyroradii) of the magnetic field in forging the early-on particle acceleration at shocks.