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