On Stochastic Ion Heating in the Inner Heliosphere: Radial Trends and Parker Solar Probe Observations

Stochastic heating (SH) is a non-linear plasma heating mechanism, frequently proposed as a candidate to explain the strong heating of the solar wind ions perpendicular to the magnetic field. It is driven by the violation of magnetic moment invariance due to large-amplitude, low-frequency Alfvenic turbulent fluctuations. Using Helios and Parker Solar Probe (PSP) observations, we track the radial variation of SH throughout the inner heliosphere from 0.16 to 1 au. We find that SH is increasingly important as one observes plasma closer to the Sun, specifically that the stochastic heating rate varies as $Q_{SH} \sim r^{-2.5}$. In accordance with theoretical predictions, observations of flattop shaped proton velocity distributions are characteristic for periods where SH is predicted to be a dominant heating mechanism. We also find that $Q_{SH}$ does not significantly vary inside intermittent structures, such as switchbacks regularly measured by PSP.