Decoding the formation of hammerhead ion populations observed by Parker Solar Probe

TL;DR

This paper explains the formation of hammerhead ion populations observed by Parker Solar Probe through a proton firehose-like instability, showing how it affects proton distributions and temperature anisotropy.

Contribution

It introduces a quasi-linear theory model that links proton beam instabilities to hammerhead ion features, a novel explanation for these observations.

Findings

Proton firehose-like instability can produce hammerhead ion populations.

The instability reduces beam-core drift and induces perpendicular temperature anisotropy.

Hammerhead distributions are transient, influenced by relaxation mechanisms.

Abstract

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.