Evidence of a "current-mediated" turbulent regime in space and astrophysical plasmas

TL;DR

This study identifies a new turbulent regime in space and astrophysical plasmas characterized by a steep power-law spectrum below ion scales, driven by ion kinetic energy and magnetic fields, relevant across various plasma environments.

Contribution

The paper introduces a simple two-fluid model explaining a current-mediated turbulent regime with a steep spectral slope, supported by observations and simulations.

Findings

Power spectrum slope of -11/3 at sub-ion scales

Ion kinetic energy significantly contributes to turbulence

Model explains turbulence without kinetic or electron-inertia effects

Abstract

How the turbulent energy cascade develops below the magnetohydrodynamic scales in space and astrophysical plasmas is a major open question. Here, we measure the power spectrum of magnetic fluctuations in Parker Solar Probe's observations close to the Sun and in state-of-the-art numerical simulations of plasma turbulence. Both reveal a power-law behavior with a slope compatible with $-11/3$ at scales smaller than the ion characteristic scales, steeper than what is typically observed in the solar wind and in the Earth's magnetosheath. We explain such behavior by developing a simple two-fluid model which does not require any kinetic processes nor electron-inertia effects. This is characterized by a significant contribution of the ion kinetic energy to the total turbulent energy cascade at sub-ion scales, although the dynamics is driven by the magnetic field through the current density. We expect that this regime may be relevant for a broad class of low-beta plasmas, e.g. the solar corona, non-relativistic magnetized jets and disks, and laboratory plasmas.