Spectral properties and energy transfer at kinetic scales in collisionless plasma turbulence

Abstract

Plasma turbulence is a complex process whose properties at sub-ion scales are not yet fully understood. By means of a fully kinetic simulation of freely decaying plasma turbulence, we study the spectral properties of the turbulent cascade at kinetic scales. We find that the magnetic field spectrum follows the k −α exp(−λ k) law at kinetic scales, with an exponential range starting around k ρ e 1, consistently with satellite observations. The same exponential decay is observed in the electron velocity spectrum but not in the ion velocity spectrum, that drops like a steep power law before reaching electron scales. We investigate the dynamics responsible for the development of the exponential falloff by analyzing the high-pass filtered electromagnetic work J • E and pressure-strain interaction −P : ∇u. We find that the electrons play a major role with respect to the ions in driving the magnetic field turbulent cascade at kinetic scales, contributing to the formation of the magnetic field exponential range. Our analysis shows that at fully developed turbulence the magnetic field energy is dissipated by a two-stage mechanism involving mainly the electrons that first subtract energy from the magnetic field and then convert it into internal energy through the pressure-strain interaction at electron scales. Finally, we discuss a possible explanation for the formation of the magnetic field spectral breaks at sub-ion scales in terms of particles demagnetization.

Anisotropic Electron Heating in Turbulence-driven Magnetic Reconnection in the Near-Sun Solar Wind

“…Fully kinetic simulations of collisionless turbulent plasmas, which retain both ion and electron kinetic effects, represent an invaluable tool for investigating the turbulent energy cascade down to electron scales (e.g., Grošelj et al 2018;Cerri et al 2019;González et al 2019;Roytershteyn et al 2019), its interplay with magnetic reconnection (e.g., Karimabadi et al 2013;Pucci et al 2017Pucci et al , 2018aAdhikari et al 2021;Agudelo Rueda et al 2021), and the role of electron-scale coherent structures in dissipating energy and heating particles (e.g., Camporeale & Burgess 2011;Parashar et al 2015;Yang et al 2017;Arrò et al 2021;Bandyopadhyay et al 2021;Yang et al 2022). They have also provided numerical evidence for an enhancement of the electron parallel temperature anisotropy in the outflows of strong reconnection events, which occurred spontaneously as the result of the interactions between subproton-scale turbulent structures (Camporeale & Burgess 2011;Haynes et al 2014).…”

Observations of Cross Scale Energy Transfer in the Inner Heliosphere by Parker Solar Probe

“…The colours represent 2D histograms with darker colours representing more points. The spectra show a power-law behaviour with f −8/3 superposed with exponential decay at electron scales indicating strong damping of magnetic fluctuations [98,118,119]. Based on the above considerations, an overall view of the cascade of energy from large to small scales emerges to be as follows (see Fig.…”

Spectral features and energy cascade of kinetic scale plasma turbulence

“…Our analysis shows that the magnetic field dynamics at kinetic scales is mainly driven by the electrons that are responsible for the formation of the exponential range. In particular, we see that at fully developed turbulence the magnetic field energy is dissipated by a two-stage mechanism lead by the electrons that first subtract energy from the magnetic field and then convert it into internal energy at electron scales through the pressure-strain interaction, that accounts for the electron heating [2].…”