Abstract
Interplanetary shocks are one of the proposed sources of suprathermal ion populations (i.e., ions with energies of a few times the solar-wind energy). Here we present results from a series of 3D hybrid simulations of collisionless shocks in the solar wind. We focus on the influence of the shock-normal angle, θ
Bn
, and the shock speed, V
s
, on producing protons with energies a few to hundreds of times the thermal energy of the upstream plasma. The combined effects of θ
Bn
and V
s
result in shocks with Alfvén Mach numbers in the range 3.0–6.0 and fast magnetosonic Mach numbers in the range 2.5–5.0, representing moderate to strong interplanetary shocks. We find that θ
Bn
largely organizes the shape of proton energy spectra, while shock speed controls acceleration efficiency. All shocks accelerate protons at the shock front, but the spectral evolution depends on θ
Bn
. Shocks with θ
Bn
≥ 60° produce isolated bursts of suprathermal protons at the shock front, while shocks with θ
Bn
≤ 45° create suprathermal beams upstream of the shock. Downstream proton energy spectra have exponential or smoothed broken power-law forms when θ
Bn
≥ 45° and a single power-law form when θ
Bn
≤ 30°. Protons downstream of the strongest shocks have energies at least 100 times the upstream thermal energy, with θ
Bn
≤ 30° shocks producing the highest-energy protons and θ
Bn
≥ 60° shocks producing the largest number of protons with energies at least a few times the thermal energy.
