Multi-strand coronal loop model and filter-ratio analysis

TL;DR

This study models a coronal loop as multiple strands heated independently by Alfvén waves, explaining observed flat temperature profiles and questioning the effectiveness of triple-filter analysis in confirming isothermality.

Contribution

Introduces a multi-strand coronal loop model with differential heating, providing insights into temperature profiles and the limitations of filter-ratio diagnostics.

Findings

Flat temperature profiles result from multi-strand structure with different temperatures.

Triple-filter analysis may not reliably determine isothermality due to observational uncertainties.

Model reproduces observed EUV loop characteristics.

Abstract

We model a coronal loop as a bundle of seven separate strands or filaments. Each of the loop strands used in this model can independently be heated (near their left footpoints) by Alfv\'en/ion-cyclotron waves via wave-particle interactions. The Alfv\'en waves are assumed to penetrate the strands from their footpoints, at which we consider different wave energy inputs. As a result, the loop strands can have different heating profiles, and the differential heating can lead to a varying cross-field temperature in the total coronal loop. The simulation of TRACE observations by means of this loop model implies two uniform temperatures along the loop length, one inferred from the 171:195 filter ratio and the other from the 171:284 ratio. The reproduced flat temperature profiles are consistent with those inferred from the observed EUV coronal loops. According to our model, the flat temperature profile is a consequence of the coronal loop consisting of filaments, which have different temperatures but almost similar emission measures in the cross-field direction. Furthermore, when we assume certain errors in the simulated loop emissions (e.g., due to photometric uncertainties in the TRACE filters) and use the triple-filter analysis, our simulated loop conditions become consistent with those of an isothermal plasma. This implies that the use of TRACE/EIT triple filters for observation of a warm coronal loop may not help in determining whether the cross-field isothermal assumption is satisfied or not.