Coronal Loop Models and Those Annoying Observations!

TL;DR

Recent advances reveal that coronal loops are dynamic structures heated impulsively by nanoflares, challenging the static equilibrium model and highlighting the importance of storm duration and thermal processes.

Contribution

This paper compares impulsive nanoflare heating models with thermal nonequilibrium, clarifying their roles in explaining coronal loop observations.

Findings

Short storms produce narrow thermal distributions.

Long storms produce broad thermal distributions.

Thermal nonequilibrium is unlikely for monolithic loops but may apply to multi-stranded loops.

Abstract

It was once thought that all coronal loops are in static equilibrium, but observational and modeling developments over the past decade have shown that this is clearly not the case. It is now established that warm (~1 MK) loops observed in the EUV are explainable as bundles of unresolved strands that are heated impulsively by storms of nanoflares. A raging debate concerning the multi-thermal versus isothermal nature of the loops can be reconciled in terms of the duration of the storm. We show that short and long storms produce narrow and broad thermal distributions, respectively. We also examine the possibility that warm loops can be explained with thermal nonequilibrium, a process by which steady heating produces dynamic behavior whenever the heating is highly concentrated near the loop footpoints. We conclude that this is not a viable explanation for monolithic loops under the conditions we have considered, but that it may have application to multi-stranded loops. Serious questions remain, however.