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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 IRIS, 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 red-shifted component of comparable intensity. The red-shifted components migrate from 25-50 km s$^{-1}$ towards the rest wavelength within $\sim$30 seconds.

    Using Fermi hard X-ray observations, we derive the parameters of an accelerated electron beam impacting the dense chromosphere, using them to drive a RHD simulation with the RADYN code. As in [@2017ApJ...836...12K], our simulations show that the most energetic electrons penetrate into the deep chromosphere, heating it to T$\sim$10,000 K, while the bulk of the electrons dissipate their energy higher, driving an explosive evaporation, and its counterpart condensation — a very dense (n$_e \sim 2 \times 10^{14}$ cm$^{-3}$), thin layer (30–40 km thickness), heated to 8–12,000 K, moving towards the stationary chromosphere at up to 50 km s$^{-1}$.

    The synthetic Fe [ii]{} 2814.45Å 