Ion acceleration at the Earth's bow shock and at interplanetary shocks: A comparison

Using ACE data, we selected a number of characteristic "energetic storm particle" (ESP) events, i.e., solar energetic particles events in which the CME-driven interplanetary shock passes the spacecraft; to compare observed local proton flux profiles with those obtained from hybrid simulations (kinetic ions, electron fluid). The events were selected for undisturbed solar wind and profiles indicating local shock acceleration. Interestingly, in the sub-MeV range, we find very little variation of peak proton fluxes with shock normal angle. This is in stark contrast to the Earth's bow sbock which in both observations and simulations only shows weak reflected ion beams, but no significant turbulence or secondary ion acceleration for shock-normal angles of similar to 45 degrees or larger. We find that in sufficiently large simulations of oblique, planar shocks, even the dilute upstream ion beams suffice to generate upstream compressional waves, which change the local shock-normal angle upon impact on the shock. In a feed-back mechanism, this leads to more wave-particle interactions in now more parallel portions of the shock, and results in a shock undulation propagating along the surface, and in energetic proton fluxes that are enhanced by up to three orders of magnitude.