Nature of electrostatic fluctuations in the terrestrial magnetosheath

TL;DR

This paper investigates small-scale electrostatic fluctuations in the terrestrial magnetosheath, revealing ion-bulk wave generation linked to non-Maxwellian particle distributions and ion cyclotron waves, advancing understanding of energy transfer in space plasmas.

Contribution

It provides new insights into the origin of sub-proton scale electric field fluctuations and their association with ion-beam features and ion cyclotron waves, supported by observations and simulations.

Findings

Bumps in electric field spectral density correlate with ion-beam features.

Ion cyclotron waves are frequently observed at these locations.

Ion-bulk waves propagate at the ion thermal speed, indicating a new energy transfer mechanism.

Abstract

The high cadence plasma, electric, and magnetic field measurements by the Magnetospheric MultiScale spacecraft allow us to explore the near-Earth space plasma with an unprecedented time and spatial resolution, resolving electron-scale structures that naturally emerge from plasma complex dynamics. The formation of small-scale turbulent features is often associated to structured, non-Maxwellian particle velocity distribution functions that are not at thermodynamic equilibrium. Using measurements in the terrestrial magnetosheath, this study focuses on regions presenting bumps in the power spectral density of the parallel electric field at sub-proton scales. Correspondingly, it is found that the ion velocity distribution functions exhibit beam-like features at nearly the local ion thermal speed. Ion cyclotron waves in the ion-scale range are frequently observed at the same locations. These observations, supported by numerical simulations, are consistent with the generation of ion-bulk waves that propagate at the ion thermal speed. This represents a new branch of efficient energy transfer at small scales, which may be relevant to weakly collisional astrophysical plasmas.