Non-Thermal "Burst-on-Tail" of Long-Duration Solar Event on 26 October 2003

TL;DR

This paper analyzes a rare long-duration solar event with a delayed high-energy emission burst, suggesting changes in electron injection into larger magnetic loops cause the observed phenomena.

Contribution

It introduces the concept of a "burst-on-tail" in long-duration solar events and explains its possible origin through magnetic loop size and electron injection dynamics.

Findings

Delayed hard X-ray and microwave burst observed

No significant soft X-ray response during the burst

Larger magnetic loops involved in the tail emission

Abstract

Observations of a rare long-duration solar event of GOES class X1.2 from 26 October 2003 are presented. This event showed a pronounced burst of hard X-ray and microwave emission, which was extremely delayed (>60 min) with respect to the main impulsive phase and did not have any significant response visible in soft X-ray emission. We refer to this phenomenon as a "burst-on-tail". Based on TRACE observations of the growing flare arcade and some simplified estimation, we explain why a reaction of active region plasma to accelerated electrons may change drastically over time. We suggest that, during the "burst-on-tail", non-thermal electrons were injected into magnetic loops of larger spatial scale than during the impulsive phase bursts, thus resulting in much smaller values of plasma temperature and emission measure in their coronal volume, and hence little soft X-ray flux. The nature of the long gap between the main impulsive phase and the "burst-on-tail" is, however, still an open question.