Can Large Time Delays Observed in Light Curves of Coronal Loops be Explained by Impulsive Heating?

TL;DR

This study investigates whether impulsive heating can explain the large time delays observed in coronal loop light curves, finding it insufficient for the largest delays and suggesting alternative explanations or more complex models are needed.

Contribution

The paper demonstrates that impulsive heating alone cannot account for the largest observed time delays in coronal loops, highlighting the need for alternative models or additional physical processes.

Findings

Impulsive heating explains only small to moderate time delays.

Large time delays require long, expanding loops with photospheric abundances.

Additional observations may challenge the current simulation-based explanations.

Abstract

The light curves of solar coronal loops often peak first in channels associated with higher temperatures and then in those associated with lower. The time delays between the different narrowband EUV channels have been measured for many individual loops and recently for every pixel of an active region observation. Time delays between channels for an active region exhibit a wide range of values, with maxima $>$ 5,000\,s. These large time delays make up 3-26\% (depending on the channel pair) of the pixels where a significant, positive time delay is measured. It has been suggested that time delays can be explained by impulsive heating. In this paper, we investigate whether the largest observed time delays can be explained by this hypothesis by simulating a series of coronal loops with different heating rates, loop lengths, abundances, and geometries to determine the range of expected time delays between a set of four EUV channels. We find that impulsive heating cannot address the largest time delays observed in two of the channel pairs and that the majority of the large time delays can only be explained by long, expanding loops with photospheric abundances. Additional observations may rule out these simulations as an explanation for the long time delays. We suggest that either the time delays found in this manner may not be representative of real loop evolution, or that the impulsive heating and cooling scenario may be too simple to explain the observations and other heating scenarios must be explored.