Spectral Signatures of Chromospheric Condensation in a Major Solar Flare

Abstract

We study the evolution of chromospheric line and continuum emission during the impulsive phase of the X-class SOL2014-09-10T17:45 solar flare. We extend previous analyses of this flare to multiple chromospheric lines of Fe I, Fe II, Mg II, C I, and Si II observed with the Interface Region Imaging Spectrograph, combined with radiative-hydrodynamical (RHD) modeling. For multiple flaring kernels, the lines all show a rapidly evolving double-component structure: an enhanced emission component at rest, and a broad, highly redshifted component of comparable intensity. The redshifted components migrate from 25 to 50 km s^-1 toward the rest wavelength within ∼30 s. Using Fermi hard X-ray observations, we derive the parameters of an accelerated electron beam impacting the dense chromosphere, using them to drive an RHD simulation with the RADYN code. As in Kowalski et al. (2017), our simulations show that the most energetic electrons penetrate into the deep chromosphere, heating it to T ∼ 10,000 K, while the bulk of the electrons dissipate their energy higher, driving an explosive evaporation, and its counterpart condensation—a very dense (n_e< ∼ 2 × 10^14 cm^-3), thin layer (30-40 km thickness), heated to 8-12,000 K, moving toward the stationary chromosphere at up to 50 km s^-1. The synthetic Fe II 2814.45 Å profiles closely resemble the observational data, including a continuum enhancement, and both a stationary and a highly redshifted component, rapidly moving toward the rest wavelength. Importantly, the absolute continuum intensity, ratio of component intensities, relative time of appearance, and redshift amplitude are sensitive to the model input parameters, showing great potential as diagnostics.