Properties of the Stormtime Plasma Sheet at the Lunar Distance

TL;DR

This study analyzes ARTEMIS spacecraft data to understand plasma and field properties in the lunar magnetotail during storm and recovery phases, revealing significant electron energization and turbulence-related phenomena.

Contribution

It provides new insights into electron energization mechanisms in the lunar magnetotail during geomagnetic storms, highlighting the role of electrostatic turbulence and reconnection.

Findings

Electron temperatures increase by a factor of 4 during storm recovery.

Ion temperatures increase less than electron temperatures, reducing T_i/T_e ratio.

Electrostatic fluctuation power increases up to approximately 2 mV/m.

Abstract

The electron fluxes at energies $E>$100\,keV are shown to be vanishing in the quiet time plasma sheet at geocentric distance of 60 Earth's radii (R$_E$) where the Moon traverses the magnetotail. Fluxes of energetic electrons up to relativistic energies were, however, observed during disturbed space weather conditions. In this paper, we study the data collected by the two lunar-orbiting Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft during their magnetotail traverses at two magnetic storm events. These observations allow us to compare plasma and field properties obtained at prior to storm and during the storm, including the storm recovery phase. We found that on the storms' recovery phases the average electron temperature increased by a factor of 4 compare to the pre-storm electron temperature. The ion temperature gain, however, did not increase a factor of 2. That leads to a decrease of ion to electron temperature ration to $\langle{T_i}/{T_e}\rangle\approx$3, in contrast to the pre-storm value of 7 to 9. We also found an increase in integral power of electrostatic fluctuations up to $\approx$2\,|mV/m|. Our observations suggest that the electrons were energized to energies $E>$100\,keV in the magnetotail. Although the exact mechanism of this energization remains unclear, we suggest that energization via continuous sporadic electron-only reconnection associated with electrostatic turbulence may be responsible for the anomalous electron energization.