The role of asymmetries in coronal rain formation during thermal non-equilibrium cycles

Context. Thermal non-equilibrium (TNE) produces several observables that can be used to constrain the spatial and temporal distribution of solar coronal heating. Its manifestations include prominence formation, coronal rain, and long-period intensity pulsations in coronal loops. The recent observation of abundant periodic coronal rain associated with intensity pulsations allowed for these two phenomena to be unified as the result of TNE condensation and evaporation cycles. On the other hand, many observed intensity pulsation events show little to no coronal rain formation. Aims. Our goal is to understand why some TNE cycles produce such abundant coronal rain, while others produce little to no rain. Methods. We reconstructed the geometry of the periodic coronal rain event, using images from the Extreme Ultraviolet Imager (EUVI) onboard the Solar Terrestrial Relations Observatory (STEREO), and magnetograms from the Helioseismic and Magnetic Imager (HMI). We then performed 1D hydrodynamic simulations of this event for different heating parameters and variations of the loop geometry (9000 simulations in total). We compared the resulting behaviour to simulations of TNE cycles that do not produce coronal rain. Results. Our simulations show that both prominences and TNE cycles (with and without coronal rain) can form within the same magnetic structure. We show that the formation of coronal rain during TNE cycles depends on the asymmetry of the loop and of the heating. Asymmetric loops are overall less likely to produce coronal rain, regardless of the heating. In symmetric loops, coronal rain forms when the heating is also symmetric. In asymmetric loops, rain forms only when the heating compensates for the asymmetry.

Automated summary

By studying the geometric shape of periodic coronal rain events and conducting simulations, we found that the occurrence of abundant coronal rain or lack thereof in TNE cycles depends on the asymmetry of the loop and heating. The results show that asymmetric loops are less likely to produce coronal rain, while symmetric loops only produce rain when the heating is also symmetric.