Identification of Super- and Sub-critical Regions in Shocks driven by Coronal Mass Ejections

TL;DR

This study uses coronagraphic images to analyze CME-driven shocks, revealing how their criticality varies along the shock front and over time, impacting their ability to accelerate particles and influence space weather.

Contribution

The paper introduces a method to estimate shock compression ratios from white-light images, linking shock criticality to particle acceleration potential in CME events.

Findings

Only a small region around the shock center is supercritical early on

The entire shock becomes subcritical at higher altitudes and later times

CME shocks can efficiently accelerate particles during initiation but lose this ability over time

Abstract

In this work, we focus on the analysis of a CME-driven shock observed by SOHO/LASCO. We show that white-light coronagraphic images can be employed to estimate the compression ratio X = rho_d / rho_u all along the front of CME-driven shocks. X increases from the shock flanks (where X ~ 1.2) to the shock center (where X ~ 3.0 is maximum). From the estimated X values, we infer the Alfv\'en Mach number for the general case of an oblique shock. It turns out that only a small region around the shock center is supercritical at earlier times, while higher up in the corona the whole shock becomes subcritical. This suggests that CME-driven shocks could be efficient particle accelerators at the initiation phases of the event, while at later times they progressively loose energy, also losing their capability to accelerate high energy particles. This result has important implications on the localization of particle acceleration sites and in the context of predictive space weather studies.