Temperature anisotropy, instabilities, and thermal processes in low- <i>β</i> plasma

Interplanetary coronal mass ejections (ICMEs) are large-scale magnetic structures that influence heliospheric dynamics and space weather. While wave activity has been observed within their low- β (≪1) magnetic obstacles, the role of temperature anisotropy and instability remains underexplored. This study examines proton temperature anisotropies, heating, cooling, turbulence, and collisional effects within ICME magnetic obstacles, which are low- β plasmas. Using Wind spacecraft data from 382 ICME magnetic obstacles at 1 au (spanning 1995–2021), we observe that proton temperature and proton β p follow log-normal distributions. The anisotropy within these regions is primarily constrained to the stable parameter regime below the thresholds for the mirror-mode and oblique firehose instabilities. Additionally, plasmas unstable to proton cyclotron and firehose instabilities exhibit temperatures that are significantly higher – 50 to 100 times higher than those of stable plasma. Notably, enhanced magnetic fluctuations and low collisional age are observed near instability thresholds, regardless of beta. Although a clear relationship exists between temperature and collisional age, the correlation between turbulent amplitude and collisional age is weak, differing from trends observed in the solar wind. Our results suggest a causal chain whereby high turbulence amplitudes are associated with enhanced heating, linked to reduced collisions, causing increased temperature anisotropy, and ultimately favouring the development of instabilities within ICME magnetic obstacles.