STOCHASTIC HEATING, DIFFERENTIAL FLOW, AND THE ALPHA-TO-PROTON TEMPERATURE RATIO IN THE SOLAR WIND

Abstract

We extend previous theories of stochastic ion heating to account for the\nmotion of ions along the magnetic field. We derive an analytic expression for\nthe ion-to-proton perpendicular temperature ratio in the solar wind for any ion\nspecies, assuming that stochastic heating is the dominant ion heating\nmechanism. This expression describes how this temperature ratio depends upon\nthe average velocity of the ions along the magnetic field direction and the\nratio of the parallel proton pressure to the magnetic pressure. We compare our\nmodel with previously published measurements of alpha particles and protons\nfrom the WIND spacecraft. We find that stochastic heating offers a promising\nexplanation for these measurements when the fractional cross helicity and\nAlfven ratio at the proton-gyroradius scale have values that are broadly\nconsistent with solar-wind measurements.\n