Spontaneous Magnetic Fluctuations and Collisionless Regulation of Turbulence in the Earth's Magnetotail

TL;DR

This study demonstrates that spontaneous magnetic fluctuations and their regulation via collisionless processes are universal features of space plasmas, observed in Earth's magnetotail and solar wind, advancing understanding of plasma turbulence.

Contribution

It provides the first in-situ evidence of collisionless regulation of magnetic fluctuations in Earth's magnetotail, extending the universality of these processes beyond solar wind.

Findings

Magnetic fluctuations are regulated by kinetic instabilities.

Regulation occurs at ion and electron scales.

Results suggest a universal plasma behavior.

Abstract

Among the fundamental and most challenging problems of laboratory, space, and astrophysical plasma physics is to understand the relaxation processes of nearly collisionless plasmas toward quasi-stationary states; and the resultant states of electromagnetic plasma turbulence. Recently, it has been argued that solar wind plasma $\beta$ and temperature anisotropy observations may be regulated by kinetic instabilities such as the ion-cyclotron, mirror, electron-cyclotron, and firehose instabilities; and that magnetic fluctuation observations are consistent with the predictions of the Fluctuation-Dissipation theorem, even far below the kinetic instability thresholds. Here, using in-situ magnetic field and plasma measurements by the THEMIS satellite mission, we show that such regulation seems to occur also in the Earth's magnetotail plasma sheet at the ion and electron scales. Regardless of the clear differences between the solar wind and the magnetotail environments, our results indicate that spontaneous fluctuations and their collisionless regulation are fundamental features of space and astrophysical plasmas, thereby suggesting the processes is universal.