Universality of solar wind turbulent spectrum from MHD to electron scales

TL;DR

This study demonstrates a universal magnetic turbulence spectrum in space plasmas across MHD to electron scales, revealing consistent spectral shapes and identifying the electron Larmor radius as a key dissipation scale.

Contribution

It provides the first observation of an exponential magnetic spectrum in space plasmas and links the electron Larmor radius to turbulence dissipation.

Findings

Spectra have similar shapes across different plasma conditions.

Identified a universal spectrum with three parts: two power laws and an exponential.

Electron Larmor radius acts as the dissipation scale.

Abstract

In order to investigate the universality of magnetic turbulence in space plasmas we analyze seven time periods in the free solar wind of different origin, slow or fast, and under different plasma conditions. The orientation of magnetic field to the flow velocity was always quasi-perpendicular. Unique combination of three instruments on Cluster spacecraft which operate in different frequency ranges give us the possibility to resolve spectra up to 300 Hz. We show that spectra measured under different plasma conditions have a similar shape. Such a quasi-universal spectrum consists of three parts: two power laws and an exponential domain. At MHD scales, Kolmogorov's law $\sim k^{-5/3}$ is found. At scales smaller than the ion characteristic scales, a $k^{-2.8}$ law is observed. At scales $k\rho_e\sim (0.1-1)$, where $\rho_e$ is the electron gyroradius, the magnetic spectrum follows an exponential law $\exp(-k^{1/2})$, indicating the onset of dissipation. This is the first observation of an exponential magnetic spectrum in space plasmas. We show that among several spatial kinetic plasma scales, the electron Larmor radius plays the role of a dissipation scale in space plasma turbulence.