Solar wind turbulent spectrum at plasma kinetic scales

TL;DR

This study analyzes 100 solar wind magnetic fluctuation spectra at kinetic scales, revealing a universal spectral law with an exponential tail likely due to electron Landau damping.

Contribution

It provides the first comprehensive statistical characterization of the turbulent spectrum from ion to electron scales in the solar wind, identifying a universal spectral form.

Findings

Spectra follow a $k_ot^{-8/3}$ power law with an exponential tail.

The spectral form is consistent across different measurements.

The exponential tail suggests electron Landau damping as a damping mechanism.

Abstract

The description of the turbulent spectrum of magnetic fluctuations in the solar wind in the kinetic range of scales is not yet completely established. Here, we perform a statistical study of 100 spectra measured by the STAFF instrument on the Cluster mission, which allows to resolve turbulent fluctuations from ion scales down to a fraction of electron scales, i.e. from $\sim 10^2$ km to $\sim 300$ m. We show that for $k_{\perp}\rho_e \in[0.03,3]$ (that corresponds approximately to the frequency in the spacecraft frame $f\in [3,300]$ Hz), all the observed spectra can be described by a general law $E(k_\perp)\propto k_\perp^{-8/3}\exp{(-k_\perp \rho_e)}$, where $k_{\perp}$ is the wave-vector component normal to the background magnetic field and $\rho_e$ the electron Larmor radius. This exponential tail found in the solar wind seems compatible with the Landau damping of magnetic fluctuations onto electrons.