Probing progression of heating through the lower flare atmosphere via high-cadence IRIS spectroscopy

Recent high-cadence flare campaigns by the Interface Region Imaging Spectrograph (IRIS) have offered new opportunities to study rapid processes characteristic of flare energy release, transport, and deposition. Here, we examine high-cadence chromospheric and transition region spectra acquired by IRIS during a C-class flare from 2022 September 25. Within the flare ribbon, the intensities of the Si ɪᴠ 1402.77 Å, C ɪɪ 1334.53 Å and Mg ɪɪ k 2796.35 Å lines peaked at different times, with the transition region Si ɪᴠ typically peaking before the chromospheric Mg ɪɪ line by 1–6 s. To understand the nature of these delays, we probed a grid of radiative hydrodynamic flare simulations heated by electron beams, thermal conduction-only, or Alfvén waves. Electron beam parameters were constrained by hard X-ray observations from the Gamma-ray Burst Monitor (GBM) onboard the Fermi spacecraft. Reproducing light curves where Si ɪᴠ peaks precede those in Mg ɪɪ proved to be a challenge because only a subset of Fermi/GBM-constrained electron beam models were consistent with the observations. Light curves with relative timings consistent with the observations were found in simulations heated by either high-flux electron beams or by Alfvén waves, while the thermal conduction heating does not replicate the observed delays. Our analysis shows how delays between chromospheric and transition region emission pose tight constraints on flare models and properties of energy transport, highlighting the importance of obtaining very high-cadence data sets with IRIS and other observatories.