Conditions for Thermal Non-Equilibrium

Thermal non-equilibrium (TNE) is a fascinating phenomenon in which perfectly steady heating produces inherently dynamic behavior in coronal loops. Sometimes, but not always, a condensation of cold ( 10,000 K) material is produced. This is the likely explanation of coronal rain, prominences, and "pulsating loops," and has been suggested to have additional application in active regions and even helmet streamers. A primary requirement of TNE is that steady heating must be concentrated at low altitudes in the corona. Another requirement is that asymmetries in the heating and/or loop cross sectional area cannot be too great. We present theoretical formulas for predicting whether TNE will occur and whether the associated condensations will be full or incomplete. The predictions are compared against hydrodynamic simulations.