Letter to the Editor

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

¹ Department of Physics and Materials Sciences, College of Arts and Sciences, Qatar University, 2713 Doha, Qatar
² Centre for Mathematical Plasma Astrophysics, Dept. of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
³ Institute for Theoretical Physics IV, Faculty of Physics and Astronomy, Ruhr-University Bochum, D-44780 Bochum, Germany
⁴ Research Center in the intersection of Plasma Physics, Matter, and Complexity ( P 2 m c), Comisión Chilena de Energía Nuclear, Casilla 188-D, Santiago, Chile
⁵ Departamento de Ciencias Físicas, Facultad de Ciencias Exactas, Universidad Andres Bello, Sazié 2212, Santiago 8370136, Chile
⁶ Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742-2431, USA
⁷ Institute of Physics, University of Maria Curie-Skłodowska, Pl. M. Curie-Skłodowskj 1, 20-031 Lublin, Poland

^⋆ Corresponding author; shamd@qu.edu.qa, s.m.shaaban88@gmail.com

Received: 11 September 2024
Accepted: 13 November 2024

Abstract

Context. In situ observations by the Parker Solar Probe (PSP) have revealed new properties of the proton velocity distributions (VDs), including hammerhead features that suggest a non-isotropic broadening of the beams.

Aims. The present work proposes a very plausible explanation for the formation of hammerhead proton populations through the action of a proton firehose-like instability triggered by the proton beam.

Methods. We investigated a self-generated firehose-like instability driven by the relative drift of ion populations using a simplified moment-based quasi-linear (QL) theory. While simpler and faster than advanced numerical simulations, this toy model provided rapid insights and concisely highlighted the role of plasma micro-instabilities in relaxing the observed anisotropies of particle VDs in the solar wind and space plasmas.

Results. The QL theory proposed here shows that the resulting transverse waves are right-hand polarized and have two consequences on the protons: (i) They reduce the relative drift between the beam and the core, but above all, (ii) they induce a strong perpendicular temperature anisotropy specific to the observed hammerhead ion beam. Moreover, the long-run QL results suggest that these hammerhead distributions are rather transitory states that are still subject to relaxation mechanisms, in which instabilities such as the one discussed here are very likely involved. This foundational work motivates future detailed studies using advanced methods.