Authors: Tamar Ervin (UC Berkeley | SSL), Trevor A. Bowen (UC Berkeley | SSL), Alfred Mallet (UC Berkeley | SSL), Philip A. Isenberg (University of New Hampshire), Kristopher G. Klein (University of Arizona), Stuart D. Bale (UC Berkeley | SSL), Benjamin D. G. Chandran (University of New Hampshire), Roberto Livi (UC Berkeley | SSL), Ali Rahmati (UC Berkeley | SSL), and Davin E. Larson (UC Berkeley | SSL)

Parker Solar Probe (PSP) provides novel kinetic-scale measurements of the inner heliosphere that allow us to constrain the dissipation mechanisms responsible for the solar wind’s heating as it evolves. We use observations of the three-dimensional proton velocity distribution function (VDF), from the ion electrostatic analyzer (SPANi) instrument on PSP, to empirically estimate diffusion coefficients and heating rates as a function of velocity-space. We compare these empirical measurements with observationally constrained, analytic expressions of stochastic heating (SH) and resonant heating (RH) by parallel ion cyclotron waves (ICWs). We show that analytic expressions for SH via non-coherent fluctuations match the empirical measurements in the sub-Alfvenic wind both in amplitude, and in the region of phase-space where the heating takes place, provided that we account for intermittency in the heating calculation. In contrast, the derived heating rates for RH via parallel-ICWs and SH without the inclusion of intermittency do not. We show differences between empirically measured diffusion coefficients in the sub-Alfvenic and super-Alfvenic wind, pointing to different primary dissipation mechanisms as the solar wind expands. Our approach provides novel methodology to uniquely identify and constrain heating processes in collisionless plasmas.