Proton Temperature Anisotropy Across Interplanetary Shocks: A Statistical Analysis with WIND observations

TL;DR

This study analyzes ~800 interplanetary shocks observed by Wind from 1997-2024, revealing how shock geometry influences proton temperature anisotropy, deviations from adiabatic models, and the role of kinetic instabilities in regulation.

Contribution

It provides a comprehensive statistical analysis of proton temperature anisotropy across interplanetary shocks, highlighting geometry-dependent effects and the influence of kinetic instabilities.

Findings

Quasi-perpendicular shocks enhance perpendicular temperature downstream.

Deviations from CGL model depend on shock geometry.

Anisotropy relaxes with distance from the shock.

Abstract

Interplanetary (IP) shocks efficiently modify the proton temperature anisotropy of the solar wind. Analyzing ~800 IP shocks observed by the Wind spacecraft from 1997-2024, we present a statistical study of upstream and downstream proton temperature anisotropy and its dependence on shock geometry, compression, and distance from the shock. We find that (1) quasi-perpendicular shocks produce a pronounced enhancement of perpendicular temperature downstream (Tperp > Tpara), whereas parallel shocks remain near isotropic downstream due to typically stronger upstream Tpara; (2) comparisons with the Chew-Goldberger-Low (CGL) double-adiabatic model reveal geometry-dependent deviations. CGL overestimates downstream perpendicular heating and underestimates parallel heating at quasi-perpendicular shocks, with the opposite trend at quasi-parallel shocks, highlighting the importance of non-adiabatic processes beyond simple compression; (3) Shock-driven anisotropy is strongly localized near the shock and gradually relaxes toward typical solar wind conditions farther downstream as the shock's influence diminishes; and (4) downstream anisotropy is regulated by kinetic instabilities, with quasi-perpendicular shocks constrained by proton cyclotron and mirror instabilities and quasi-parallel shocks limited by the parallel firehose instability. Together, these results show that the evolution of temperature anisotropy at interplanetary shocks is controlled by shock geometry, localized processes, and instability driven regulation.