Occurrence of Non-Stationarity at Earth's Quasi-Perpendicular Bow Shock

TL;DR

This study statistically demonstrates that Earth's quasi-perpendicular bow shock is predominantly non-stationary, with phase-space holes indicating shock reformation and surface ripples, especially at higher Mach numbers.

Contribution

It provides the first large-scale statistical analysis of phase-space hole occurrence at Earth's bow shock, highlighting the prevalence of non-stationarity.

Findings

65% of bow shock crossings show phase-space holes.

Occurrence peaks at 70% for Mach numbers >7.

Non-stationarity is common at Earth's quasi-perpendicular bow shock.

Abstract

Collisionless shocks can exhibit non-stationary behavior even under steady upstream conditions, forming a complex transition region. Ion phase-space holes, linked to shock self-reformation and surface ripples, are a signature of this non-stationarity. We statistically analyze their occurrence using 521 crossings of Earth's quasi-perpendicular bow shock. Phase-space holes appear in 65% of cases, though the actual rate may be higher as the holes may not be resolved during fast shock crossings. The occurrence rate peaks at 70% for shocks with Alfv\'en Mach numbers $M_A>7$. These findings suggest that Earth's quasi-perpendicular bow shock is predominantly non-stationary.