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 ${\epsilon }_p$. Enhanced proton temperatures are observed when ${\epsilon }_p$ is larger than a critical value ($\sim 0.019-0.025$). This enhancement strongly depends on the proton plasma beta (${\beta }_{||p}$); when ${\beta }_{||p}\ll 1$ only the perpendicular proton temperature $T_{\perp }$ increases, while for ${\beta }_{||p}\sim 1$ increased parallel and perpendicular proton temperatures are both observed. For ${\epsilon }_p$ smaller than the critical value and ${\beta }_{||p}\ll 1$ no enhancement of $T_p$ is observed while for ${\beta }_{||p}\sim 1$ 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 ${\epsilon }_p>{\epsilon }_{\mathrm{crit}}$ 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

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