Abstract
For some forms of steady heating, coronal loops are in a state of thermal nonequilibrium and evolve in a manner that includes accelerated cooling, often resulting in the formation of a cold condensation. This is frequently confused with thermal instability, but the two are in fact fundamentally different. We explain the distinction and discuss situations where they may be interconnected. Large-amplitude perturbations, perhaps associated with MHD waves, likely play a role in explaining phenomena that have been attributed to thermal nonequilibrium but also seem to require cross-field communication.
Highlights
Thermal nonequilibrium (TNE) is a fascinating property that may explain a number of solar phenomena, including coronal rain, prominence formation, long-period loop pulsations, and quasi-periodic disturbances in the solar wind
Thermal nonequilibrium describes a state of the plasma contained in a magnetic flux tube that is rooted to the solar surface at both ends, i.e. a coronal loop
We have argued that the formation of a condensation during a TNE cycle cannot be formally classified as a thermal instability because there is no equilibrium to go unstable
Summary
Thermal nonequilibrium (TNE) is a fascinating property that may explain a number of solar phenomena, including coronal rain, prominence formation, long-period loop pulsations, and quasi-periodic disturbances in the solar wind. Theoretical modeling reveals that steady coronal heating usually results in an equilibrium, where the plasma does not evolve. Something very interesting can occur when steady heating is concentrated in the low corona If it decreases sufficiently rapidly with distance from the loop footpoints, no equilibrium is possible. Higher densities cause stronger radiation, and the loop cools, slowly at first, but at an ever increasing rate This usually culminates in the formation of a cold (chromospheric temperature) high-density condensation. Thermal nonequilibrium is an example of a limit cycle, in which the system (loop) retraces the same path within phase space (e.g. temperature versus density). The sequence of evolution is : TNE conditions in the full loop → condensation formation in the magnetic dip → steady flow conditions in the two “half loops.”. The sequence of evolution is : TNE conditions in the full loop → condensation formation in the magnetic dip → steady flow conditions in the two “half loops.” There is a single condensation cycle even though the heating and geometry never change
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