The Thermal Instability of Solar Prominence Threads

TL;DR

This paper investigates thermal instability as a potential cause for the short-lived appearance of solar prominence threads, modeling them as magnetic tubes with inhomogeneous plasma conditions and analyzing their stability through magnetohydrodynamic equations.

Contribution

It introduces a combined analytical and numerical approach to study thermal instability in prominence threads, highlighting the role of the PCTR in their rapid disappearance.

Findings

Thermal modes are unstable in the PCTR above 80,000 K.

Instability time scale is a few minutes, independent of temperature profile.

Thermal instability may explain the short lifetimes of prominence threads.

Abstract

The fine structure of solar prominences and filaments appears as thin and long threads in high-resolution images. In H-alpha observations of filaments, some threads can be observed for only 5 - 20 minutes before they seem to fade and eventually disappear, suggesting that these threads may have very short lifetimes. The presence of an instability might be the cause of this quick disappearance. Here, we study the thermal instability of prominence threads as an explanation of their sudden disappearance from H-alpha observations. We model a prominence thread as a magnetic tube with prominence conditions embedded in a coronal environment. We assume a variation of the physical properties in the transverse direction, so that the temperature and density continuously change from internal to external values in an inhomogeneous transitional layer representing the particular prominence-corona transition region (PCTR) of the thread. We use the nonadiabatic and resistive magnetohydrodynamic equations, which include terms due to thermal conduction parallel and perpendicular to the magnetic field, radiative losses, heating, and magnetic diffusion. We combine both analytical and numerical methods to study linear perturbations from the equilibrium state, focusing on unstable thermal solutions. We find that thermal modes are unstable in the PCTR for temperatures higher than 80,000 K, approximately. These modes are related to temperature disturbances that can lead to changes in the equilibrium due to rapid plasma heating or cooling. For typical prominence parameters, the instability time scale is of the order of a few minutes and is independent of the form of the temperature profile within the PCTR of the thread. This result indicates that thermal instability may play an important role for the short lifetimes of threads in the observations.