The Role of Asymmetries in Thermal Non-Equilibrium

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.

arXiv comment38 pages, 9 figures, 2 tables, accepted by Astrophysical Journal

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• Electron Beams Cannot Directly Produce Coronal Rain
• Self-consistent 3D radiative MHD simulations of coronal rain formation and evolution
• The influence of flux rope heating models on solar prominence formation
• Modelling the solar transition region using an adaptive conduction method
• Coronal Condensation as the Source of Transition Region Supersonic Downflows above a Sunspot
• Spatial and Temporal Analysis of Quiescent Coronal Rain over an Active Region
• Prevalence of Thermal Non-Equilibrium over an Active Region
• EUV fine structure and variability associated with coronal rain revealed by Solar Orbiter/EUI HRIEUV and SPICE
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• Simulation of Thermal Nonequilibrium Cycles in the Solar Wind
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• Construction of coronal hole and active region magnetohydrostatic solutions in two dimensions: Force and energy balance
• The Equilibrium of Coronal Loops Near Separatrices
• A fast multi-dimensional magnetohydrodynamic formulation of the transition region adaptive conduction (TRAC) method
• Thermal Non-equilibrium Cycles in a Non-eruptive Pseudo-Streamer