Spectroscopic Observations of Coronal Rain Formation and Evolution Following an X2 Solar Flare

Abstract

A significant impediment to solving the coronal heating problem is that we currently only observe active region loops in their cooling phase. Previous studies showed that the evolution of cooling loop densities and apex temperatures is insensitive to the magnitude, duration, and location of energy deposition. Still, potential clues to how energy is released are encoded in the properties of the cooling phase. The appearance of coronal rain, one of the most spectacular phenomena of the cooling phase, occurs when plasma has cooled below 1 MK, which sets constraints on the heating frequency, for example. Most observations of coronal rain have been made by imaging instruments. Here we report rare Hinode/EUV Imaging Spectrometer (EIS) observations of a loop arcade where coronal rain forms following an X2.1 limb flare. A bifurcation in plasma composition measurements between photospheric at 1.5 MK and coronal at 3.5 MK suggests that we are observing postflare-driven coronal rain. Increases in nonthermal velocities and densities with decreasing temperature (2.7–0.6 MK) suggest that we are observing the formation and subsequent evolution of the condensations. Doppler velocity measurements imply that a 10% correction of apparent flows in imaging data is reasonable. Emission measure analysis at 0.7 MK shows narrow temperature distributions, indicating coherent behavior reminiscent of that observed in coronal loops. The limitations on spatio-temporal resolution of EIS suggest that we are observing the largest features or rain showers. These observations provide insights into the heating rate, source, turbulence, and collective behavior of coronal rain from observations of the loop cooling phase.