The 26 December 2001 Solar Eruptive Event Responsible for GLE63. III. CME, Shock Waves, and Energetic Particles

TL;DR

This study analyzes a significant solar eruptive event from December 2001, revealing how multiple shocks and prior eruptions contributed to high-energy particle acceleration and ground-level cosmic-ray enhancements.

Contribution

It provides a detailed multi-wavelength analysis of shock formation, eruption interactions, and particle acceleration mechanisms in a major solar event, highlighting the role of preceding eruptions.

Findings

Two shock waves merged into a single interplanetary shock.

The shock wave accelerated particles during the flare rise.

Preceding eruptions enhanced particle escape and acceleration.

Abstract

The SOL2001-12-26 moderate solar eruptive event (GOES importance M7.1, microwaves up to 4000 sfu at 9.4 GHz, CME speed 1446 km/s) produced strong fluxes of solar energetic particles and ground-level enhancement of cosmic-ray intensity (GLE63). To find a possible reason for the atypically high proton outcome of this event, we study multi-wavelength images and dynamic radio spectra and quantitatively reconcile the findings with each other. An additional eruption probably occurred in the same active region about half an hour before the main eruption. The latter produced two blast-wave-like shocks during the impulsive phase. The two shock waves eventually merged around the radial direction into a single shock traced up to $25R_\odot$ as a halo ahead of the expanding CME body, in agreement with an interplanetary Type II event recorded by the Radio and Plasma Wave Investigation (WAVES) experiment on the Wind spacecraft. The shape and kinematics of the halo indicate an intermediate regime of the shock between the blast wave and bow shock at these distances. The results show that i) the shock wave appeared during the flare rise and could accelerate particles earlier than usually assumed; ii) the particle event could be amplified by the preceding eruption, which stretched closed structures above the developing CME, facilitated its lift-off and escape of flare-accelerated particles, enabled a higher CME speed and stronger shock ahead; iii) escape of flare-accelerated particles could be additionally facilitated by reconnection of the flux rope, where they were trapped, with a large coronal hole; iv) the first eruption supplied a rich seed population accelerated by a trailing shock wave.