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We present NUV flare spectra from the Hubble Space Telescope/Cosmic OriginsSpectrograph during two moderate-amplitude U-band flares on the dM4e star GJ1243. These spectra are some of the first accurately flux-calibrated, NUV flarespectra obtained over the impulsive phase in M dwarf flares. We observed theseflares with a fleet of nine ground-based telescopes simultaneously, whichprovided broadband photometry and low-resolution spectra at the Balmer jump. Abroadband continuum increase occurred with a signal-to-noise > 20 in the HSTspectra, while numerous Fe II lines and the Mg II lines also increased but withsmaller flux enhancements compared to the continuum radiation. These two eventsproduced the most prominent Balmer line radiation and the largest Balmer jumpsthat have been observed to date in dMe flare spectra. A T=9000 K blackbodyunder-estimates the NUV continuum flare flux by a factor of two and is a poorapproximation to the white-light in these types of flare events. Instead, ourdata suggest that the peak of the specific continuum flux density isconstrained to U-band wavelengths near the Balmer series limit. Aradiative-hydrodynamic simulation of a very high energy deposition rateaveraged over times of impulsive heating and cooling better explains thelam>2500 Angstrom flare continuum properties. These two events sample only oneend of the empirical color-color distribution for dMe flares, and moretime-resolved flare spectra in the NUV, U-band, and optical from lam=2000-4200Angstrom are needed during more impulsive and/or more energetic flares.

The Near-Ultraviolet Continuum Radiation in the Impulsive Phase of HF/GF-Type dMe Flares I: Data