Stochastic heating of the solar wind becomes increasingly significant close to the Alfven critical point

Stochastic heating, a non-linear heating mechanism driven by the violation of magnetic moment invariance due to large-amplitude turbulent fluctuations producing energy diffusion perpendicular to the magnetic field, is frequently invoked as a mechanism responsible for the heating of ions in the solar wind. Here, we quantify for the first time the proton stochastic heating rate Q_⊥ at radial distances down to 0.16 au, using measurements from the first two Parker Solar Probe encounters. We find satisfying agreement for both the amplitude and radial trend of the heating rate, Q_⊥ ∼ r^-2.5, calculated using the Helios data set at distances from 0.3 to 0.9 au. In agreement with previous results, Q_⊥ is significantly larger in the fast compared to slow solar wind. We identify the tendency in fast solar wind of cuts of the core proton velocity distribution transverse to the magnetic field to exhibit a flat-top shape. The observed distribution agrees with previous theoretical predictions for fast solar wind where stochastic heating is the dominant heating mechanism.