The First Ground Level Enhancement Event of Solar Cycle 24: Direct Observation of Shock Formation and Particle Release Heights

TL;DR

This paper reports the first ground level enhancement event of Solar Cycle 24, providing direct observations of shock formation and particle release heights, and compares it with non-GLE eruptions to understand the conditions for particle acceleration.

Contribution

It offers the first direct measurement of shock formation and particle release heights during a GLE event in Solar Cycle 24, enhancing understanding of CME-driven shocks and particle acceleration.

Findings

Shock formation height at 1.38 Rs from Sun center.

Particle release height at 2.32 Rs, consistent with CME kinematics.

Connectivity issues due to CME motion and solar B0 angle affect GLE occurrence.

Abstract

We report on the 2012 May 17 Ground Level Enhancement (GLE) event, which is the first of its kind in Solar Cycle 24. This is the first GLE event to be fully observed close to the surface by the Solar Terrestrial Relations Observatory (STEREO) mission. We determine the coronal mass ejection (CME) height at the start of the associated metric type II radio burst (i.e., shock formation height) as 1.38 Rs (from the Sun center). The CME height at the time of GLE particle release was directly measured from a STEREO image as 2.32 Rs, which agrees well with the estimation from CME kinematics. These heights are consistent with those obtained for cycle-23 GLEs using back-extrapolation. By contrasting the 2012 May 17 GLE with six other non-GLE eruptions from well-connected regions with similar or larger flare size and CME speed, we find that the latitudinal distance from the ecliptic is rather large for the non-GLE events due to a combination of non-radial CME motion and unfavorable solar B0 angle, making the connectivity to Earth poorer. We also find that the coronal environment may play a role in deciding the shock strength.