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

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 (STA) on 2008 March 9^th. 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.