Snow Flakes in the Oven - Cool Material in the Hot Solar Corona and the Thermal Instability

The Sun's outer atmosphere, the corona, is million-degrees hot and tenuous. Such hot plasma, under certain conditions, can enigmatically undergo a radiative cooling instability and condense into material of 100 times cooler in the form of coronal rain or so-called solar prominences. Where, when, and how such cooling condensation takes place remain poorly understood. Magnetic fields in the magnetized corona undoubtedly play a crucial role (e.g., by trapping the plasma), but where and how? We report recent imaging and spectroscopic observations from NASA's Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS) missions that can shed light on this puzzle. Through a systematic survey, we found that a large fraction of quiet-Sun condensations preferentially occur at the dips of coronal loops or funnels. Such dips are located at/near magnetic topological features, such as null points and quasi-separatrix layers (QSLs), which are regions characterized by high values of the squashing factor. We also identified evidence of magnetic reconnection at such locations, which can produce favorable conditions, e.g., density enhancement by compression and/or mass trapping in plasmoids, that can trigger run-away radiative cooling. We present proof-of-concept MHD simulations that demonstrate the role of reconnection in transporting cooled mass from overlying, long loops to underlying, short loops where it slides down as coronal rain. We will discuss the significance and broader implications of these results beyond the Sun, elsewhere in the Universe and in laboratories.