Parallel-cascade-based mechanisms for heating solar coronal loops: test against observations

TL;DR

This paper tests a parallel-cascade heating mechanism for solar coronal loops against observations, finding it explains some soft X-ray loops but not ultraviolet loops, suggesting different heating processes may be involved.

Contribution

It extends the parallel-cascade model to coronal loops, incorporating loop expansion and comparing results with observations to evaluate its effectiveness.

Findings

The model accounts for some soft X-ray loops but predicts too high temperatures for ultraviolet loops.

Lowering wave amplitudes does not resolve temperature discrepancies.

Turbulence-based steady heating may explain ultraviolet loops, while nanoflares may explain soft X-ray loops.

Abstract

The heating of solar coronal loops is at the center of the problem of coronal heating. Given that the origin of the fast solar wind has been tracked down to atmospheric layers with transition region or even chromospheric temperatures, it is worthy attempting to address whether the mechanisms proposed to provide the basal heating of the solar wind apply to coronal loops as well. We extend the loop studies based on a classical parallel-cascade scenario originally proposed in the solar wind context by considering the effects of loop expansion, and perform a parametric study to directly contrast the computed loop densities and electron temperatures with those measured by TRACE and YOHKOH/SXT. This comparison yields that with the wave amplitudes observationally constrained by SUMER measurements, while the computed loops may account for a significant fraction of SXT loops, they seem too hot when compared with TRACE loops. Lowering the wave amplitudes does not solve this discrepancy, introducing magnetic twist will make the comparison even less desirable. We conclude that the nanoflare heating scenario better explains ultraviolet loops, while turbulence-based steady heating mechanisms may be at work in heating a fraction of soft X-ray loops.