  The 2014 March 29 X1 solar flare (SOL20140329T17:48) produced bright
continuum emission in the far- and near-ultraviolet (NUV) and highly asymmetric
chromospheric emission lines, providing long-sought constraints on the heating
mechanisms of the lower atmosphere in solar flares. We analyze the continuum
and emission line data from the Interface Region Imaging Spectrograph (IRIS) of
the brightest flaring magnetic footpoints in this flare. We compare the NUV
spectra of the brightest pixels to new radiative-hydrodynamic predictions
calculated with the RADYN code using constraints on a nonthermal electron beam
inferred from the collisional thick-target modeling of hard X-ray data from
RHESSI. We show that the atmospheric response to a high beam flux density
satisfactorily achieves the observed continuum brightness in the NUV. The NUV
continuum emission in this flare is consistent with hydrogen (Balmer)
recombination radiation that originates from low optical depth in a dense
chromospheric condensation and from the stationary beam-heated layers just
below the condensation. A model producing two flaring regions (a condensation
and stationary layers) in the lower atmosphere is also consistent with the
asymmetric Fe II chromospheric emission line profiles observed in the impulsive
phase.
