The Role of Asymmetries in Thermal Non-Equilibrium

TL;DR

This paper investigates the conditions under which thermal non-equilibrium occurs in coronal loops, emphasizing the roles of asymmetries in heating and geometry, and provides formulae to predict TNE occurrence validated by simulations.

Contribution

It introduces two formulae to quantify the conditions for TNE, highlighting the importance of asymmetries and steady heating in coronal loops, supported by hydrodynamic simulations.

Findings

A ratio of apex to footpoint heating less than 0.1 favors TNE.

Asymmetries less than about a factor of 3 are conducive to TNE.

The formulae agree well with 1D hydrodynamic simulations.

Abstract

Thermal non-equilibrium (TNE) is a fascinating situation that occurs in coronal magnetic flux tubes (loops) for which no solution to the steady-state fluid equations exists. The plasma is constantly evolving even though the heating that produces the hot temperatures does not. This is a promising explanation for isolated phenomena such as prominences, coronal rain, and long-period pulsating loops, but it may also have much broader relevance. As known for some time, TNE requires that the heating be both (quasi) steady and concentrated at low coronal altitudes. Recent studies indicate that asymmetries are also important, with large enough asymmetries in the heating and/or cross-sectional area resulting in steady flow rather than TNE. Using reasonable approximations, we have derived two formulae for quantifying the conditions necessary for TNE. As a rough rule of thumb, the ratio of apex to footpoint heating rates must be less than about 0.1, and asymmetries must be less than about a factor of 3. The precise values are case dependent. We have tested our formulae with 1D hydrodynamic loop simulations and find a very acceptable agreement. These results are important for developing physical insight about TNE and assessing how widespread it may be on the Sun.