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 October 3 (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. One-dimensional 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 near-UV (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 ≈ 5 × 10 14 cm −3 and optical depths τ ≈ 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 far-UV (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 ≈ 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.