Properties of Magnetic Field Fluctuations in the Earth’s Magnetotail and Implications for the General Problem of Structure Formation in Hot Plasmas

Abstract

In this review we discuss the formation of plasma structures in hot plasmas with large $\beta$. We use spacecraft observations of magnetic field fluctuations in the Earth magnetotail to reveal the main multiscale plasma properties. Fourier spectra of magnetic field fluctuations observed by various spacecraft in the different domains of the magnetotail at quiet or moderately disturbed times demonstrated a number of practically universal features: (1) the presence of two kinks at low (${\sim}5\times 10^{-2}~\mathrm{Hz}$) and high (${\sim}5\times 10^{-1}~\mathrm{Hz}$) frequencies, (2) in the frequency interval between the kinks the power law shape with an index ${\sim }2.5$, and (3) the significant intensification of fluctuations with the increase of plasma flow velocities. To describe these spectra we consider properties of a principal structure supporting the equilibrium of a hot plasma configuration—the magnetotail current sheet. The observed spatial scales of current sheets are often very small and almost reach the ion Larmor radius. Convection of such mesoscale plasma structures by plasma flows can be responsible for the formation of a certain part of the spectrum of magnetic field fluctuations. Lower- and higher- frequency domains in Fourier spectra can be generated respectively by large-scale MHD oscillations of current sheets and by kinetic small-scale instabilities excited by strong local magnetic field gradients existing in active current sheets.