The Atmospheric Response to Large Electron Beam Fluxes in Solar Flares III: Comprehensive Modeling of the Brightest Observed Near-Ultraviolet Continuum Source in an X9 Solar Flare
Adam F. Kowalski
TL;DR
This study tests high-flux electron-beam heating scenarios in solar flares by confronting 1D RADYN RHD models with comprehensive IRIS and CHASE observations of a remarkable X9 flare. The analysis shows that dense chromospheric condensations with electron densities around $n_e \approx 5\times10^{14}$ cm$^{-3}$ can reproduce extreme NUV continua and Hα near-wing broadening, but fail to match the brightness of Fe II red-wing components, the detailed Fe I/II line evolution, and the observed NUV-to-FUV continuum ratios, implying faster-than-realistic cooling or missing physics in the models. The work highlights the need for improved beam transport physics, opacities, and possibly multi-dimensional radiative-hydrodynamic treatments to capture the full spectral evolution. It also underscores a solar–stellar flare discrepancy, as solar kernels remain brighter in NUV relative to FUV than some stellar megaflare spectra, guiding future cross-discipline refinements. Overall, while high-flux beam models explain several salient kernel properties, precise reproduction of all spectral diagnostics requires advances in radiative transfer and condensations modeling beyond current 1D RADYN implementations.
Abstract
I report on the high resolution spectra of the remarkable X9 solar flare of 2024 Oct 03 (SOL2024-10-03T12:08) and evaluate the extent to which nonthermal electron beams that generate dense chromospheric condensations can power very bright kernels in solar flares. 1D Radiative-hydrodynamic models predict extreme H$α$ near-wing broadening, bright continuum intensities, and a rapid Fe II red wing asymmetry evolution at the brightest NUV continuum source in the flare. Detailed comparisons to the spectral observations reveal that the H$α$ line is too broad, the Fe II red wing is too bright, and the NUV continuum decays too slowly in a fiducial high-flux beam model. However, chromospheric condensations with maximum electron densities of $n_e \approx 5 \times 10^{14}$ cm$^{-3}$ and optical depths $τ\approx 1$ in the near wing of H$α$ are consistent with the observed intensity of a broad spectrum in the Southern ribbon. Model similarities demonstrate that Fe I emission lines and the FUV continuum intensity can form at chromospheric heights during flares, but I find that the ratios of the NUV to FUV continuum intensities are generally too large in the models. This suggests that radiative-hydrodynamic models of chromospheric condensations cool through $T \approx 30,000$ K too rapidly. The larger than expected FUV continuum intensities are not nearly bright enough to explain recent stellar megaflare spectra from the Hubble Space Telescope.