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Register a new userNature of stochastic ion heating in the solar wind: testing the dependence on plasma beta and turbulence amplitude
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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 $epsilon_p > epsilon_{rm crit}$ in 76% of the studied periods implying that stochastic heating may operate most of the time in the solar wind at 1 AU.
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Evidence for inhomogeneous heating in the interplanetary plasma near current sheets dynamically generated by magnetohydrodynamic (MHD) turbulence is obtained using measurements from the ACE spacecraft. These coherent structures only constitute 19% of the data, but contribute 50% of the total plasma internal energy. Intermittent heating manifests as elevations in proton temperature near current sheets, resulting in regional heating and temperature enhancements extending over several hours. The number density of non-Gaussian structures is found to be proportional to the mean proton temperature and solar wind speed. These results suggest magnetofluid turbulence drives intermittent dissipation through a hierarchy of coherent structures, which collectively could be a significant source of coronal and solar wind heating.
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