Electron Beams Cannot Directly Produce Coronal Rain

Abstract

Coronal rain is ubiquitous in flare loops, forming shortly after the onset of the solar flare. Rain is thought to be caused by a thermal instability, a localized runaway cooling of material in the corona. The models that demonstrate this require extremely long duration heating on the order of the radiative cooling time, localized near the footpoints of the loops. In flares, electron beams are thought to be the primary energy transport mechanism, driving strong footpoint heating during the impulsive phase that causes evaporation, filling and heating flare loops. Electron beams, however, do not act for a long period of time, and even supposing that they did, their heating would not remain localized at the footpoints. With a series of numerical experiments, we show directly that these two issues mean that electron beams are incapable of causing the formation of rain in flare loops. This result suggests that either there is another mechanism acting in flare loops responsible for rain, or that the modeling of the cooling of flare loops is somehow deficient. To adequately describe flares, the standard model must address this issue to account for the presence of coronal rain.

Global Energetics of Solar Flares. XI. Flare Magnitude Predictions of the GOES Class

“…The GOES flux appears to correlate with the upper limit of the absolute total flux near the strong-field, high-gradient polarity inversion lines (Schrijver 2007;. A strong correlation has been established between the GOES flux and the thermal energy (Reep et al 2013(Reep et al , 2020, or between the temperature as well as the emission measure of the thermal plasma and the GOES flux (Warmuth et al 2016a;2016b). Even for cool (small) flares of GOES class B5 to C2, the emission measure was found to be correlated with the GOES flux (Phillips and Feldman 1995).…”

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

“…In addition to particle beams and in-situ heating followed by thermal conduction, it has also been suggested that Alfvén waves may play a role in transporting energy from flare sites and depositing or thermalizing this magnetic free energy in the lower atmosphere (Fletcher and Hudson, 2008;Russell and Fletcher, 2013), but observational evidence has been sparse. Numerical work has shown that the heating from electron beams alone cannot account for coronal rain, a common occurrence in flaring loops during their last cooling stage (Reep, Antolin, and Bradshaw, 2020). Such work suggests the need for an additional, long-lasting (radiative-cooling timescale) and footpoint-concentrated heating mechanism triggered by the flare, and future work needs to consider whether Alfvén waves fit the purpose.…”

Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST)

“…Finally, issues involving the origin, amplitude, and stability of the return currents required to support these dense coronal beams are not fully resolved (Alaoui and Holman, 2017). Together these considerations suggest that additional modes of energy transport may be required, as do very recent modeling results indicating that electron beams are incapable of producing coronal rain, which is often found shortly after the onset of loop flares (Reep, Antolin, and Bradshaw, 2020). Proposed additional transport mechanisms include ion beams (Hurford et al, 2006;Zharkova and Zharkov, 2007), heat conduction (Longcope, 2014;Graham, Fletcher, and Labrosse, 2015;Longcope, Qiu, and Brewer, 2016), and Alfvén waves (Russell and Fletcher, 2013;Reep and Russell, 2016;Reep et al, 2018).…”