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Past observations show that solar wind (SW) acceleration occurs inside thesub-Alfvenic region, reaching the local Alfven speed at typical distances ~ 10- 20 Rs (solar radii). Recently, Parker Solar Probe (PSP) traversed regions ofsub-Alfvenic SW near perihelia in encounters E8-E12 for the first timeproviding data in these regions. It became evident that properties of themagnetically dominated SW are considerably different from the super-Alfvenicwind. For example, there are changes in relative abundances and drift of alphaparticles with respect to protons, as well as in the magnitude of magneticfluctuations. We use data of the magnetic field from the FIELDS instrument, andconstruct ion velocity distribution functions (VDFs) from the sub-Alfvenicregions using Solar Probe Analyzer Ions (SPAN-I) data, and run 2.5D and 3Dhybrid models of proton-alpha sub-Alfvenic SW plasma. We investigate thenonlinear evolution of the ion kinetic instabilities in several case studies,and quantify the transfer of energy between the protons, alpha particles, andthe kinetic waves. The models provide the 3D ion VDFs at the various stages ofthe instability evolution in the SW frame. By combining observational analysiswith the modeling results, we gain insights on the evolution of the ioninstabilities, the heating and the acceleration processes of the sub-AlfvenicSW plasma and quantify the exchange of energy between the magnetic and kineticcomponents. The modeling results suggest that the ion kinetic instabilities areproduced locally in the SW, resulting in anisotropic heating of the ions, asobserved by PSP.

Observations and Modeling of Unstable Proton and Alpha Particle Velocity Distributions in Sub-Alfvenic Solar Wind at PSP Perihelia