When Hot Meets Cold: Post-flare Coronal Rain

Wenzhi Ruan,Rony Keppens,Yuhao Zhou

doi:10.3847/2041-8213/ac27b0

Abstract

Abstract Most solar flares demonstrate a prolonged, hour-long 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 (∼30 MK) and dense plasma that 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 ∼1 MK regimes, much cooler (∼0.01 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 the 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 analyze 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.

Highlights

Solar flares represent explosive phenomena in the solar atmosphere, where 1028 −1032 ergs of energy originally stored in the solar magnetic field can suddenly be released via magnetic reconnection (Sweet 1958; Shibata & Magara 2011)
We find that the runaway cooling and rain formation induces the appearance of dark post-flare loop systems, as observed in extreme ultraviolet (EUV) channels
The role of cool and dense coronal plasma in EUV emission and absorption leading to dark post-flare loops (DPFLs) has been emphasized previously (Jejcic et al 2018; Heinzel et al 2020), with coronal rain observations (Scullion et al 2014; Martınez Oliveros et al 2014; Jing et al 2016; Scullion et al 2016) and our simulation results showing how this cool and dense plasma can be generated in post-flare loops (PFLs)

Summary

INTRODUCTION

Solar flares represent explosive phenomena in the solar atmosphere, where 1028 −1032 ergs of energy originally stored in the solar magnetic field can suddenly be released via magnetic reconnection (Sweet 1958; Shibata & Magara 2011). The loops return to their usual coronal conditions (∼1 MK, ∼ 108 − 109 cm−3) in the following, gradual phase, where field-guided thermal conduction and radiative losses generally contribute to the cooling process (Cargill et al 1995; Aschwanden & Alexander 2001) These loops, visible in extreme ultraviolet (EUV), and in Hα images, are usually called post-flare loops (PFLs) (Bruzek 1964). We perform an MHD simulation of a flare event from its pre-flare phase all the way into the gradual phase This simulation allows us to understand the complex thermodynamic evolutions of PFLs. We (1) reproduce postflare coronal rain; (2) quantify the chromosphere-corona mass and energy cycles during PFLs; and (3) demonstrate the intricate relationship between condensations and the disappearing EUV loops, or DPFLs

METHODS

POST-FLARE LOOP FORMATION AND EVOLUTION

MASS AND ENERGY CYCLES DURING THE GRADUAL PHASE

CONCLUSION AND DISCUSSION