Nature of stochastic ion heating in the solar wind: testing the dependence on plasma beta and turbulence amplitude

Abstract

The solar wind undergoes significant heating as it propagates away from the Sun; the exact mechanisms responsible for this heating are not yet fully understood. We present for the first time a statistical test for one of the proposed mechanisms, stochastic ion heating. We use the amplitude of magnetic field fluctuations near the proton gyroscale as a proxy for the ratio of gyroscale velocity fluctuations to perpendicular (with respect to the magnetic field) proton thermal speed, defined as $ε_p$. Enhanced proton temperatures are observed when $ε_p$ is larger than a critical value ($\sim 0.019 - 0.025$). This enhancement strongly depends on the proton plasma beta ($β_{||p}$); when $β_{||p} \ll 1$ only the perpendicular proton temperature $T_{\perp}$ increases, while for $β_{||p} \sim 1$ increased parallel and perpendicular proton temperatures are both observed. For $ε_p$ smaller than the critical value and $β_{||p} \ll 1$ no enhancement of $T_p$ is observed while for $β_{||p} \sim 1$ minor increases in $T_{\parallel}$ are measured. The observed change of proton temperatures across a critical threshold for velocity fluctuations is in agreement with the stochastic ion heating model of Chandran et al. (2010). We find that $ε_p > ε_{\rm crit}$ in 76\% of the studied periods implying that stochastic heating may operate most of the time in the solar wind at 1 AU.

Accepted for publication in The Astrophysical Journal Letters