  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.
