In situ observations by the Parker Solar Probe (PSP) have revealed new properties of the proton velocity distributions, including hammerhead features that suggest non-isotropic broadening of the beams. The present work proposes a very plausible explanation for the formation of these populations through the action of a proton firehose-like instability triggered by the proton beam. The quasi-linear (QL) theory proposed here shows that the resulting right-hand (RH) waves have two consequences on the protons: (i) reduce the relative drift between the beam and the core, but above all, (ii) induce a strong perpendicular temperature anisotropy, specific to the observed hammerhead ion strahl. Moreover, the long-run QL results suggest that these hammerhead distributions are rather transitory states, still subject to relaxation mechanisms, of which instabilities like the one discussed here are very likely involved.
@article{arxiv.2409.01997,
title = {Decoding the formation of hammerhead ion populations observed by Parker Solar Probe},
author = {Shaaban M. Shaaban and M. Lazar and R. A. López and P. H. Yoon and S. Poedts},
journal= {arXiv preprint arXiv:2409.01997},
year = {2024}
}