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The GOES X1 flare SOL2014-10-25T17:08:00 was a three-ribbon solar flareobserved with IRIS in the near and far ultraviolet. One of the flare ribbonscrossed a sunspot umbra, producing a dramatic, $\sim1000$% increase in thenear-ultraviolet (NUV) continuum radiation. We comprehensively analyze theultraviolet spectral data of the umbral flare brightenings, which provide newchallenges for radiative-hydrodynamic modeling of the chromospheric velocityfield and the white-light continuum radiation. The emission line profiles inthe umbral flare brightenings exhibit redshifts and profile asymmetries, butthese are significantly smaller than in another, well-studied X-class solarflare. We present a ratio of the NUV continuum intensity to the Fe II 2814.45Ang intensity. This continuum-to-line ratio is a new spectral diagnostic ofsignificant heating at high column mass (log $m/$[g cm$^{-2}] >-2$) duringsolar flares because the continuum and emission line radiation originate fromrelatively similar temperatures but moderately different optical depths. Thefull spectral readout of these IRIS data also allow for a comprehensive surveyof the flaring NUV landscape: in addition to many lines of Fe II and Cr II, weidentify a new solar flare emission line, He I $\lambda2829.91$ (as previouslyidentified in laboratory and early-type stellar spectra). The Fermi/GBM hardX-ray data provide inputs to radiative-hydrodynamic models (which will bepresented in Paper II) in order to better understand the largecontinuum-to-line ratios, the origin of the white-light continuum radiation,and the role of electron beam heating in the low atmosphere.

Spectral Evidence for Heating at Large Column Mass in Umbral Solar Flare Kernels I: IRIS NUV Spectra of the X1 Solar Flare of 2014 Oct 25