Ion temperature anisotropy and instability in the plasma sheet

The spatial evolution of ion temperature anisotropy and instabilities in Earth’s magnetotail from near-Earth to lunar distances has not been adequately explored. In this paper, using THEMIS-C/ARTEMIS-P2 observations, we examine the instability thresholds for different plasma flow speeds near-Earth and at lunar distances of the PS. We also compare how these thresholds vary and how ion anisotropies T⊥/T∥ regulate plasma dynamics through the onset of firehose, mirror, and cyclotron instabilities near-Earth and at lunar distances for different flow speeds. We demonstrate that flow speed and location influence all instabilities; for fast flows and for parallel plasma beta ( β ∥ >~3 ), ions have stronger parallel anisotropy and a higher firehose instability occurrence rate at lunar distance than near-Earth. We also find that most ions flowing at slow speeds ( $$< 10$$ km s ⁻¹ ) near-Earth and at lunar distances are stable. The results reveal frequent intervals, where the plasma approaches or exceeds kinetic instability thresholds, indicating active wave–particle coupling in the high- β regime. Furthermore, we find that firehose instabilities are dominant, while most ion data points are stable in the mirror and cyclotron modes during the PS intervals. These findings demonstrate that the distant magnetotail operates as a natural laboratory for anisotropy-driven instabilities, linking microscale kinetic physics to the large-scale structure of Earth’s extended magnetosphere.