Alpha/proton magnetosonic instability in the solar wind

The average relative flow velocity between protons and alpha particles in collisionless plasmas can excite several distinct alpha/proton instabilities. Here linear theory and two‐dimensional hybrid simulations in a homogeneous plasma model are used to study one such mode, the alpha/proton magnetosonic instability, using plasma parameters characteristic of the high‐speed solar wind observed by the Ulysses spacecraft at high heliospheric latitudes. Wave‐particle scattering by enhanced fluctuations from this mode reduces the alpha/proton relative speed and heats the alphas more strongly than the protons. Post‐saturation results for the alpha/proton flow in the simulations are approximately bounded by the linear theory threshold condition for onset of the magnetosonic instability, which is also a statistical upper bound on the alpha/proton relative speed at high β p for some of the Ulysses observations. Thus it is likely that this instability is a constraining agent for the alpha/proton relative speed in some domains of the solar wind.