When hot meets cold: post-flare coronal rain

Abstract

Most solar flares demonstrate a prolonged, hourlong post-flare (or gradual) phase, characterized by arcade-like, post-flare loops (PFLs) visible in many extreme ultraviolet (EUV) passbands. These coronal loops are filled with hot -- $\sim 30\mathrm{MK}$ -- and dense plasma, evaporated from the chromosphere during the impulsive phase of the flare, and they very gradually recover to normal coronal density and temperature conditions. During this gradual cooling down to $\sim 1\mathrm{MK}$ regimes, much cooler -- $\sim 0.01\mathrm{MK}$ -- and denser coronal rain is frequently observed inside PFLs. Understanding PFL dynamics in this long-duration, gradual phase is crucial to the entire corona-chromosphere mass and energy cycle. Here we report a simulation in which a solar flare evolves from pre-flare, over impulsive phase all the way into its gradual phase, which successfully reproduces post-flare coronal rain. This rain results from catastrophic cooling caused by thermal instability, and we analyse the entire mass and energy budget evolution driving this sudden condensation phenomenon. We find that the runaway cooling and rain formation also induces the appearance of dark post-flare loop systems, as observed in EUV channels. We confirm and augment earlier observational findings, suggesting that thermal conduction and radiative losses alternately dominate the cooling of PFLs.

accepted by ApJL

References in corpus (11)

• The multi-thermal and multi-stranded nature of coronal rain
• Magnetohydrodynamic shocks in and above post-flare loops: two-dimensional simulation and a simplified model
• Coronal rain in magnetic arcades: Rebound shocks, Limit cycles, and Shear flows
• Unresolved fine-scale structure in solar coronal loop-tops
• A fully self-consistent model for solar flares
• Observing the formation of flare-driven coronal rain
• Coronal rain in magnetic bipolar weak fields
• Transition region adaptive conduction (TRAC) in multidimensional magnetohydrodynamic simulations
• Self-consistent 3D radiative MHD simulations of coronal rain formation and evolution
• Electron Beams Cannot Directly Produce Coronal Rain
• Signatures of Helium continuum in cool flare loops observed by SDO/AIA