Universality of Solar-Wind Turbulent Spectrum from MHD to Electron Scales

TLDR

The study investigates the universality of magnetic turbulence in the solar wind by analyzing seven time periods under varying plasma conditions. Using three Cluster spacecraft instruments covering a broad frequency range, the authors resolve magnetic spectra up to 300 Hz. The spectra exhibit a quasi‑universal form: a Kolmogorov‑like k^−5/3 law at MHD scales, a k^−2.8 law at ion scales, and an exponential decay exp[−√(kρ_e)] at electron‑scale kρ_e≈0.1–1, marking the first observation of such an exponential magnetic spectrum and indicating dissipation with the electron Larmor radius as the dissipation scale.

Abstract

To investigate the universality of magnetic turbulence in space plasmas, we analyze seven time periods in the free solar wind under different plasma conditions. Three instruments on Cluster spacecraft operating in different frequency ranges give us the possibility to resolve spectra up to 300 Hz. We show that the spectra form a quasiuniversal spectrum following the Kolmogorov's law approximately k(-5/3) at MHD scales, a approximately k(-2.8) power law at ion scales, and an exponential approximately exp[-sqrt[k(rho)e]] at scales k(rho)e approximately [0.1,1], where rho(e) is the electron gyroradius. This is the first observation of an exponential magnetic spectrum in space plasmas that may indicate the onset of dissipation. We distinguish for the first time between the role of different spatial kinetic plasma scales and show that the electron Larmor radius plays the role of a dissipation scale in space plasma turbulence.