Strong Prevalence of Hammerhead Velocity Distributions Close to the Heliospheric Current Sheet

Abstract The solar wind undergoes nonadiabatic heating as it travels away from the Sun. The velocity phase-space distribution of nonequilibrium ions in the solar wind indicates a source of free energy that could contribute significantly to this heating. Parker Solar Probe (PSP) has observed velocity distributions containing highly anisotropic, perpendicularly diffused proton beams with a distinctly constricted gap between the core and beam populations. These distributions resemble a “hammerhead” shape and were first reported in the fourth PSP encounter. Numerical simulations have reproduced the qualitative nature of hammerheads under certain initial conditions but have not convincingly captured the prevalence or extreme attributes of the observed beam. This necessitates a broad study of the occurrence conditions and the associated plasma processes, to better guide simulations. We statistically investigate the occurrences of these structures from 20 recent PSP encounters and find that hammerheads dominantly occur around the heliospheric current sheet (HCS). As the inclination of the HCS at PSP crossing points increases over the rising phase of the solar cycle, the occurrence of hammerheads is increasingly concentrated in narrow time periods around the HCS crossings. For comparison with previous work, we present statistical trends in the anisotropy of the proton beam and its connection to the density of proton beams as well as the drift speed of the beam to the core. Our study establishes a consistent occurrence pattern of hammerhead distributions around the HCS, indicating hammerheads are diagnostics of energization processes associated with the HCS and its escaping wind.