Deriving the properties of coronal pressure fronts in 3-D: application to the 17 May 2012 ground level enhancement

TL;DR

This study introduces a novel 3-D imaging technique to analyze coronal shock properties during the 17 May 2012 GLE, revealing the shock's evolution, Mach number distribution, and particle release timing with implications for space weather understanding.

Contribution

The paper presents a new multipoint imaging method for 3-D shock analysis, integrating magnetic reconstructions and MHD simulations to better understand shock dynamics and particle acceleration.

Findings

Highest Mach numbers near the coronal neutral line.

Particle release coincides with the shock becoming super-critical.

Magnetic connectivity established via a prior eruptive magnetic cloud.

Abstract

We study the link between an expanding coronal shock and the energetic particles measured near Earth during the Ground Level Enhancement (GLE) of 17 May 2012. We developed a new technique based on multipoint imaging to triangulate the 3-D expansion of the shock forming in the corona. It uses images from three vantage points by mapping the outermost extent of the coronal region perturbed by the pressure front. We derive for the first time the 3-D velocity vector and the distribution of Mach numbers, $M_{FM}$, of the entire front as a function of time. Our approach uses magnetic field reconstructions of the coronal field, full magneto-hydrodynamic simulations and imaging inversion techniques. We find that the highest $M_{FM}$ values appear near the coronal neutral line within a few minutes of the Coronal Mass Ejection (CME) onset; this neutral line is usually associated with the source of the heliospheric current and plasma sheet. We illustrate the variability of the shock speed, shock geometry and Mach number along different modeled magnetic field lines. Despite the level of uncertainty in deriving the shock Mach numbers, all employed reconstruction techniques show that the release time of GeV particles occurs when the coronal shock becomes super-critical ($M_{FM}>3$). Combining in-situ measurements with heliospheric imagery, we also demonstrate that magnetic connectivity between the accelerator (the coronal shock of 17 May 2012) and the near-Earth environment is established via a magnetic cloud that erupted from the same active region roughly five days earlier.