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

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$\alpha$ 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$\alpha$ 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 $\tau \approx 1$ in the near wing of H$\alpha$ 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.