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

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 Alfvén Mach numbers $M_A>7$. These findings suggest that Earth's quasi-perpendicular bow shock is predominantly non-stationary.