Super- and Sub-critical Regions in Shocks driven by Radio-Loud and Radio-Quiet CMEs

TL;DR

This study compares the shock properties of radio-loud and radio-quiet CMEs, revealing that radio-loud shocks are initially super-critical and capable of particle acceleration, while radio-quiet shocks remain sub-critical and less efficient.

Contribution

It provides the first detailed analysis of the criticality states of CME-driven shocks in relation to their radio emissions and particle acceleration potential.

Findings

Radio-loud CME shock is initially super-critical and later becomes sub-critical.

Radio-quiet CME shock remains sub-critical throughout the event.

Super-critical shocks are associated with efficient particle acceleration.

Abstract

White-light coronagraphic images of Coronal Mass Ejections (CMEs) observed by SOHO/LASCO C2 have been used to estimate the density jump along the whole front of two CME-driven shocks. The two events are different in that the first one was a "radio-loud" fast CME, while the second one was a "radio quiet" slow CME. From the compression ratios inferred along the shock fronts, we estimated the Alfv\'en Mach numbers for the general case of an oblique shock. It turns out that the "radio-loud" CME shock is initially super-critical around the shock center, while later on the whole shock becomes sub-critical. On the contrary, the shock associated with the "radio-quiet" CME is sub-critical at all times. This suggests that CME-driven shocks could be efficient particle accelerators at the shock nose only at the initiation phases of the event, if and when the shock is super-critical, while at later times they lose their energy and the capability to accelerate high energetic particles.