Ion Acceleration By Foreshock Bubbles

Foreshock bubbles (FBs) form as a result of the interaction between solar wind discontinuities and backstreaming ion beams in the foreshock. They are carried by the solar wind in the anti-sunward direction and are associated with high energy ions. We employ electromagnetic hybrid (kinetic ions, fluid electrons) simulations and test particle calculations to investigate ion acceleration by FBs. Simulation results show that the maximum energies which the ions attain are not sensitive to the topology of the FBs, which varies from spherical to planar limits. However, the solar wind velocity does control the maximum energies reached by FB acceleration. Test particle calculations show that ion energization takes place through reflection of sunward moving ions from FBs through one or more collisions consistent with second-order Fermi acceleration. The results also indicate that energy gain does not reach that expected for elastic reflections. Examination of ion energy spectra within different FBs shows a self-similar pattern consisting of distribution peaked at the solar wind ram energy E-sw with a distribution tail that reaches energies of similar to 5.6 E-sw. This behavior is consistent with second-order Fermi acceleration and the inherent limitations on energy gain in this process. We also discuss the role of nonlinear structures called foreshock cavitons in the acceleration process.