Numerical simulations of temperature anisotropy instabilities stimulated by suprathermal protons

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

^⋆ Corresponding author; s.m.shaaban88@gmail.com

Received: 29 November 2023
Accepted: 19 August 2024

Abstract

Context. The new in situ measurements of the Solar Orbiter mission contribute to the knowledge of the suprathermal populations in the solar wind, especially of ions and protons whose characterization, although still in the early phase, seems to suggest a major involvement in the interaction with plasma wave fluctuations.

Aims. Recent studies point to the stimulating effect of suprathermal populations on temperature anisotropy instabilities in the case of electrons already being demonstrated in theory and numerical simulations. Here, we investigate anisotropic protons, addressing the electromagnetic ion-cyclotron (EMIC) and the proton firehose (PFH) instabilities.

Methods. Suprathermal populations enhance the high-energy tails of the Kappa velocity (or energy) distributions measured in situ, enabling characterization by contrasting to the quasi-thermal population in the low-energy (bi-)Maxwellian core. We use hybrid simulations to investigate the two instabilities (with ions or protons as particles and electrons as fluid) for various configurations relevant to the solar wind and terrestrial magnetosphere.

Results. The new simulation results confirm the linear theory and its predictions. In the presence of suprathermal protons, the wave fluctuations reach increased energy density levels for both instabilities and cause faster and/or deeper relaxation of temperature anisotropy. The magnitude of suprathermal effects also depends on each instability’s specific (initial) parametric regimes.

Conclusions. These results further strengthen the belief that wave-particle interactions govern space plasmas. These provide valuable clues for understanding their dynamics, particularly the involvement of suprathermal particles behind the quasi-stationary non-equilibrium states reported by in situ observations.