Solar Wind Turbulence and the Role of Ion Instabilities

TL;DR

This paper reviews solar wind turbulence across multiple scales, highlighting observational properties, the role of ion instabilities, and the current gaps in understanding the dissipation mechanisms in astrophysical plasmas.

Contribution

It provides a comprehensive overview of observational features of solar wind turbulence from MHD to electron scales, emphasizing the role of ion instabilities and the lack of a unified model.

Findings

Turbulence cascade develops mainly perpendicular to the magnetic field.

Spectra show a break at ion scales with increased compressibility.

Fluctuations near electron scales exhibit exponential cut-off indicating dissipation.

Abstract

Solar wind is probably the best laboratory to study turbulence in astrophysical plasmas. In addition to the presence of magnetic field, the differences with neutral fluid isotropic turbulence are: weakness of collisional dissipation and presence of several characteristic space and time scales. In this paper we discuss observational properties of solar wind turbulence in a large range from the MHD to the electron scales. At MHD scales, within the inertial range, turbulence cascade of magnetic fluctuations develops mostly in the plane perpendicular to the mean field. Solar wind turbulence is compressible in nature. The spectrum of velocity fluctuations do not follow magnetic field one. Probability distribution functions of different plasma parameters are not Gaussian, indicating presence of intermittency. At the moment there is no global model taking into account all these observed properties of the inertial range. At ion scales, turbulent spectra have a break, compressibility increases and the density fluctuation spectrum has a local flattening. Around ion scales, magnetic spectra are variable and ion instabilities occur as a function of the local plasma parameters. Between ion and electron scales, a small scale turbulent cascade seems to be established. Approaching electron scales, the fluctuations are no more self-similar: an exponential cut-off is usually observed indicating an onset of dissipation. The nature of the small scale cascade and a possible dissipation mechanism are still under debate.