Flare Impulsive-phase Durations

TL;DR

This paper discusses the relationship between impulsive-phase durations of solar flares and the timing delays between X-ray emissions, proposing a model where particle acceleration occurs near the loop top and is affected by plasma density.

Contribution

It offers a new explanation linking flare timing delays to electron beam heating duration and plasma rise time, based on observations and inferred physical processes.

Findings

Delay times depend on magnetic loop length.

Particle acceleration likely occurs near the loop top.

Heating duration influences the rise time of heated plasma.

Abstract

This Research Note is in response to the recent paper by S. M. Perriyil et al. (2026). They provide measurements of the time delay (delta t) between the hard X-ray and soft X-ray peak times for 96 flares observed with RHESSI and GOES. These delays are found to be dependent on the length of 9 the magnetic loop(s) joining the HXR footpoints seen in RHESSI images. We offer a possible explanation for this coincidence in terms of the duration of the electron beam heating, commonly inferred from the duration of the HXR emission, and the time taken for heated plasma to rise to the loop top as inferred in this paper from delta t. We suggest that the particle acceleration occurs at or near the top of the loop(s) and that it is quenched by the increase in density as the heated plasma reaches the acceleration site.