Influence of Cross-sectional Expansion on Coronal Emissions from a Radiatively Cooling Solar Flare Loop

TL;DR

This study models how cross-sectional expansion in solar flare loops influences their cooling and emission characteristics, revealing effects on EUV late-phase emissions and providing diagnostic insights.

Contribution

It introduces analytical and numerical models accounting for cross-sectional expansion, highlighting its impact on loop cooling, emissions, and observational signatures.

Findings

Cross-sectional expansion suppresses plasma draining from the corona.

Loop radiative outputs shift toward lower temperatures with expansion.

Expanded loops emit more at middle temperatures, affecting late-phase emissions.

Abstract

Loop-aligned hydrodynamic modelings help better understand the thermodynamic evolution of flaring plasma confined in solar flare loops. Conventional loop modelings typically assume a uniform loop cross section. With a variation of the cross section taken into account, in this work we carry out both analytical and numerical modelings of the radiative cooling in a solar flare loop. It is found that a cross-sectional expansion with height can efficiently suppress the draining of loop material from the corona while not significantly affecting the decrease of loop temperature. Reflected to the loop energetics, the coronal part of the loop cools more dominantly by radiation, and more importantly, the loop radiative outputs are shifted toward lower temperatures. These findings pose important physical implications for extreme-ultraviolet (EUV) late-phase emissions discovered in some solar flares. The late-phase loops in these flares are believed to bear a more notable cross-sectional expansion owing to their longer lengths. Compared with the main-phase loops, the late-phase loops would emit more effectively at middle temperatures, which could, to a certain degree, mitigate the severe heating requirement for the production of a prominent warm coronal late-phase peak. In addition, the cross-sectional expansion also affects the shape of the emission lights curves, causing a sharper decay after the emission peak. Such an emission pattern has been validated with the observations of an EUV late-phase flare, and could serve as a potential diagnostic tool to judge the degree of loop cross-sectional expansion in an extended flare dataset.