Can Thermal Nonequilibrium Explain Coronal Loops?

TL;DR

This study investigates whether thermal nonequilibrium can explain coronal loop observations by using hydrodynamic simulations, finding partial success but also significant limitations that challenge the steady heating model.

Contribution

The paper evaluates the ability of thermal nonequilibrium models to reproduce coronal loop properties, highlighting their successes and failures in matching observations.

Findings

Reproduces excess density in loops

Monolithic models show excessive intensity structure

Multi-strand models are either too structured or too long-lived

Abstract

Any successful model of coronal loops must explain a number of observed properties. For warm (~ 1 MK) loops, these include: 1. excess density, 2. flat temperature profile, 3. super-hydrostatic scale height, 4. unstructured intensity profile, and 5. 1000--5000 s lifetime. We examine whether thermal nonequilibrium can reproduce the observations by performing hydrodynamic simulations based on steady coronal heating that decreases exponentially with height. We consider both monolithic and multi-stranded loops. The simulations successfully reproduce certain aspects of the observations, including the excess density, but each of them fails in at least one critical way. Monolithic models have far too much intensity structure, while multi-strand models are either too structured or too long-lived. Our results appear to rule out the widespread existence of heating that is both highly concentrated low in the corona and steady or quasi-steady (slowly varying or impulsive with a rapid cadence). Active regions would have a very different appearance if the dominant heating mechanism had these properties. Thermal nonequilibrium may nonetheless play an important role in prominences and catastrophic cooling events (e.g., coronal rain) that occupy a small fraction of the coronal volume. However, apparent inconsistencies between the models and observations of cooling events have yet to be understood.