Nature of Stochastic Ion Heating in the Solar Wind: Testing the Dependence on Plasma Beta and Turbulence Amplitude

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 e p. Enhanced proton temperatures are observed when e p is larger than a critical value (∼0.019-0.025). This enhancement strongly depends on the proton plasma beta (b|| p); when b|| p ∼ 1 only the perpendicular proton temperature T∼ increases, while for b|| p ∼ 1 increased parallel and perpendicular proton temperatures are both observed. For e p smaller than the critical value and b|| p ∼ 1 no enhancement of Tp is observed, while for b|| p ∼ 1 minor increases in TP 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. We find that e p > Ecrit in 76% of the studied periods, implying that stochastic heating may operate most of the time in the solar wind at 1 au.