Authors: Leon Ofman (CUA/NASA GSFC), Yogesh (CUA/NASA GSFC), Scott A Boardsen (UMBC/NASA GSFC), Lan K Jian (NASA GSFC), Viacheslav M Sadykov (GSU), Parisa Mostafavi (JHU/APL), Kristopher Klein (UA/LPL), and Mihailo Martinovic (UA/LPL)

Recent measurements of solar wind protons and alpha particles from the SPAN-I instruments on the Parker Solar Probe (PSP) during solar encounters reveal anisotropic, non-Maxwellian ion velocity distributions. These distributions include core populations, beams, and the so-called ‘hammerhead’ ion populations characterized by extended anisotropic proton strahl subject to kinetic instabilities. These unstable non-Maxwellian VDFs are linked to right-hand (RH) and left-hand (LH) polarized ion-scale kinetic wave activity identified by the PSP/FIELDS instrument. We study the unstable VDFs using linearized Vlasov dispersion analysis, nonlinear 2.5D and 3D hybrid particle-in-cell (PIC) models, and Machine Learning (ML) tools. We model a range of unstable ion VDF parameters motivated by PSP observations, and investigate the formation and evolution of ion beams, and the anisotropic ion heating. The hybrid models show the rapid growth and evolution (relative to proton gyroperiods) of combined ion-cyclotron (IC) and magnetosonic (MS) instabilities, which generate LH and RH ion-scale wave spectra, respectively. The resulting nonlinear ion VDFs exhibit qualitative agreement with PSP/SPAN-I observations of the anisotropic core and ‘hammerhead’ beam velocity distributions. This study concludes that the interactions between ion-scale waves and particles associated with ion-driven kinetic instabilities represent an important stage in the dissipation of ‘free’ nonthermal energy leading to the observed heating of anisotropic solar wind plasma.