Modeling the effects of flare energy release and transport through chromospheric condensation and ultraviolet coronal emission

Abstract

Solar flares arise from the release of magnetic free energy through reconnection. A fraction of this energy travels from the corona to the lower solar atmosphere, heating the plasma and driving downflows -- chromospheric condensations -- critical to our understanding of flare energetics. While flare models with impulsive energy injections have successfully reproduced observed chromospheric responses, they typically focus on heating via electron beam deposition, neglecting other modes of energy transport. Observations of long-duration coronal emission in the extreme ultraviolet have further indicated a two-phase energy release process: impulsive energy deposition followed by persistent low-rate heating. As flare energy release and transport are measured by the indirect signatures of condensation and coronal emissions, flare models must account for these phenomena' behavior to infer the characteristics of reconnection. We first investigated the chromospheric response to a constant flare energy flux using a thermal flare model driven by in-situ coronal heating. An analytical expression for the condensation velocity was developed and found to be well described by the observed characteristic properties, allowing condensation to serve as a diagnostic for both the energy flux at the reconnection site and the pre-flare density scale height of the chromosphere. These results were tested on condensations observed in Si IV 1403 ?A spectral line redshifts. A Gaussian heating profile, inferred from footpoint UV emission corresponding to the measured downflows, was used to drive a one-dimensional simulation from which Si IV spectra were synthesized. Although the synthetic velocity evolution agreed reasonably well with observation, thus providing evidence for our model's validity, the condensation's timescale was found to be independent of the time scale of the energy release. To address coronal EUV emission signatures, long-duration flare heating was modeled through the slow dissipation of turbulent Alfven waves. Motivated by observations of supra-arcade downflows, the waves were initiated by retracting newly-reconnected flux tubes through a current sheet and dissipated through their non-linear interaction. EUV lightcurves synthesized from simulation results reproduced emissions that decayed in 40 minutes. This model, created self-consistently from reconnection-powered flare energy release, offers a possible explanation for the outstanding problem of persistent flare emission.