Electromagnetic proton/proton instabilities in the solar wind

Electromagnetic proton/proton instabilities are excited by the relative streaming parallel to the magnetic field of two distinct proton components, the more dense core and the more tenuous beam. Here linear Vlasov theory is used to study these instabilities in a homogeneous plasma model. Under conditions often observed in the high-speed solar wind, both magnetosonic and Alfven modes become proton/proton unstable; for various parameter domains there are two unstable regimes of the magnetosonic mode and three unstable regimes of the Alfven mode. In a dimensionless parameter model representing typical high-speed solar wind conditions, the most strongly unstable modes are the magnetosonic instability with maximum growth rate in the direction of the background magnetic field and the Alfven mode at propagation oblique to that field. Although the former mode has been regarded by several previous researchers to be the dominant proton/proton instability in the solar wind, the results described here indicate that the strongly unstable regime of the Alfven mode, which previously has not been studied in a solar wind model, has the lower threshold at sufficiently large beam density and/or sufficiently small core beta. Earlier studies of proton/proton instabilities in the solar wind based on the assumption that the magnetosonic mode is most important may need reconsideration.