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The solar wind undergoes significant heating as it propagates away from theSun; the exact mechanisms responsible for this heating are not yet fullyunderstood. We present for the first time a statistical test for one of theproposed mechanisms, stochastic ion heating. We use the amplitude of magneticfield fluctuations near the proton gyroscale as a proxy for the ratio ofgyroscale velocity fluctuations to perpendicular (with respect to the magneticfield) proton thermal speed, defined as $\epsilon_p$. Enhanced protontemperatures are observed when $\epsilon_p$ is larger than a critical value($\sim 0.019 - 0.025$). This enhancement strongly depends on the proton plasmabeta ($\beta_{||p}$); when $\beta_{||p} \ll 1$ only the perpendicular protontemperature $T_{\perp}$ increases, while for $\beta_{||p} \sim 1$ increasedparallel and perpendicular proton temperatures are both observed. For$\epsilon_p$ smaller than the critical value and $\beta_{||p} \ll 1$ noenhancement of $T_p$ is observed while for $\beta_{||p} \sim 1$ minor increasesin $T_{\parallel}$ are measured. The observed change of proton temperaturesacross a critical threshold for velocity fluctuations is in agreement with thestochastic ion heating model of Chandran et al. (2010). We find that$\epsilon_p > \epsilon_{\rm crit}$ in 76\% of the studied periods implying thatstochastic heating may operate most of the time in the solar wind at 1 AU.

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