Temperature anisotropy, instabilities, and thermal processes in low-β plasma

¹ William B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, TX, USA
² Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, UK
³ Department of Physics & Astronomy and Bartol Research Institute, University of Delaware, 104 The Green, Newark, DE 19716, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it. , This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 13 May 2025
Accepted: 3 November 2025

Abstract

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.