Physical Conditions of Coronal Plasma at the transit of a Shock driven by a Coronal Mass Ejection

TL;DR

This study uses combined white light and radio data to map the physical parameters of a coronal shock driven by a CME, revealing the evolution of shock properties and magnetic field strength in the solar corona.

Contribution

It presents the first 2D mapping of coronal magnetic field strength using a CME-driven shock, integrating WL coronagraphic images and radio spectra.

Findings

Shock surface is super-Alfvénic at 2-4 R☉ and only at the nose at higher altitudes.

The shock's properties correlate with the source of the type-II radio burst.

First 2D map of coronal magnetic field strength over a 10 R☉ range.

Abstract

We report here on the determination of plasma physical parameters across a shock driven by a Coronal Mass Ejection using White Light (WL) coronagraphic images and Radio Dynamic Spectra (RDS). The event analyzed here is the spectacular eruption that occurred on June 7th 2011, a fast CME followed by the ejection of columns of chromospheric plasma, part of them falling back to the solar surface, associated with a M2.5 flare and a type-II radio burst. Images acquired by the SOHO/LASCO coronagraphs (C2 and C3) were employed to track the CME-driven shock in the corona between 2-12 R$_\odot$ in an angular interval of about 110$^\circ$. In these intervals we derived 2-Dimensional (2D) maps of electron density, shock velocity and shock compression ratio, and we measured the shock inclination angle with respect to the radial direction. Under plausible assumptions, these quantities were used to infer 2D maps of shock Mach number $M_\text{A}$ and strength of coronal magnetic fields at the shock's heights. We found that in the early phases (2-4 R$_\odot$) the whole shock surface is super-Alfv\'enic, while later on (i.e. higher up) it becomes super-Alfvenic only at the nose. This is in agreement with the location for the source of the observed type-II burst, as inferred from RDS combined with the shock kinematic and coronal densities derived from WL. For the first time, a coronal shock is used to derive a 2D map of the coronal magnetic field strength over a 10 R$_\odot$ altitude and $\sim 110^\circ$ latitude intervals.