Electromagnetic proton/proton instabilities in the solar wind: Simulations

Proton velocity distributions in the high-speed solar wind are sometimes ob served as two components of approximately equal temperature with an average relative drift velocity parallel to the background magnetic field. This re lative drift gives rise to several proton/proton instabilities; for represe ntative parameters, linear Vlasov theory demonstrates that the electromagne tic modes most likely to grow are a magnetosonic instability and an Alfven instability. Two-dimensional initial value hybrid simulations of both insta bilities are carried out in a homogeneous plasma. Enhanced fluctuating fiel ds from both instabilities yield a reduction in the relative drift speed an d a characteristic heating of the more tenuous component temperature perpen dicular to the background magnetic field. Ensembles of simulations yield co nstraints on the fluctuating field energy density, the relative drift speed , and component anisotropies. Some of these constraints are cast as concise analytic expressions which may be indicators of proton/proton instability activity in the solar wind.