Pressure-strain interaction in plasma turbulence: Contribution of the ion non-gyrotropy

¹ Astronomical Institute of the Czech Academy of Sciences, Prague, Czech Republic
² Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague, Czech Republic
³ Università di Firenze – Dipartimento di Fisica e Astronomia, Largo E. Fermi 2, I-50125 Firenze, Italy
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
⁵ INFN – Sezione di Firenze, Via G. Sansone 1, I-50019 Sesto Fiorentino (FI), Italy

^⋆ Corresponding author: petr.hellinger@asu.cas.cz

Received: 5 August 2025
Accepted: 20 October 2025

Abstract

Aims. We investigated the properties of plasma turbulence at ion scales in the context of the solar wind. We concentrated on the pressure-strain coupling between the kinetic and magnetic energy and the internal energy; we analysed its capability to produce an effectively irreversible transfer towards the internal energy.

Methods. We studied results from a three-dimensional hybrid simulation of decaying turbulence when protons exhibit a substantial temperature anisotropy. We analysed the time evolution and behaviour of the combined (magnetic plus kinetic) energy and its spectral properties. Using the Kármán-Howarth-Monin (KHM) formalism, we quantified the role of the dissipation via the resistive channel and that of the pressure-strain term in generating internal energy.

Results. The combined energy flows from large to intermediate and small scales, where it is efficiently dissipated via the resistive term and is exchanged with the internal energy through the pressure-strain term. The pressure-strain coupling oscillates strongly, and this oscillation reflects its reversibility properties that are embedded in a secular evolution towards a global increase in the plasma internal energy. All the terms involved in the KHM energy balance equation are strongly anisotropic with respect to the mean magnetic field. They tend to be elongated along the mean magnetic field and oscillate over time at large scales, which is connected with the pressure-strain coupling. The reversible oscillatory part of the pressure-strain coupling is mostly contained in the gyrotropic pressure-strain part. This mainly affects the turbulent processes at large scales, but when it is time averaged, it also contributes to the ion energisation approximately at ion scales. The non-gyrotropic pressure-strain part does not oscillate significantly, acts at ion scales, and can be considered as the main effective dissipation channel.