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

Abstract

The GOES X1 flare SOL2014-10-25T17:08:00 was a three-ribbon solar flare observed with IRIS in the near and far ultraviolet. One of the flare ribbons crossed a sunspot umbra, producing a dramatic, $\sim 1000$% increase in the near-ultraviolet (NUV) continuum radiation. We comprehensively analyze the ultraviolet spectral data of the umbral flare brightenings, which provide new challenges for radiative-hydrodynamic modeling of the chromospheric velocity field and the white-light continuum radiation. The emission line profiles in the umbral flare brightenings exhibit redshifts and profile asymmetries, but these are significantly smaller than in another, well-studied X-class solar flare. We present a ratio of the NUV continuum intensity to the Fe II 2814.45 Ang intensity. This continuum-to-line ratio is a new spectral diagnostic of significant heating at high column mass (log $m/$[g cm${}^{-2}]>-2$) during solar flares because the continuum and emission line radiation originate from relatively similar temperatures but moderately different optical depths. The full spectral readout of these IRIS data also allow for a comprehensive survey of the flaring NUV landscape: in addition to many lines of Fe II and Cr II, we identify a new solar flare emission line, He I $\lambda 2829.91$ (as previously identified in laboratory and early-type stellar spectra). The Fermi/GBM hard X-ray data provide inputs to radiative-hydrodynamic models (which will be presented in Paper II) in order to better understand the large continuum-to-line ratios, the origin of the white-light continuum radiation, and the role of electron beam heating in the low atmosphere.

19 pg, 10 fig, 2 appendices. Accepted for publication in ApJ

References in corpus (20)

• Extreme ultraviolet frequency comb metrology
• The Confined X-class Flares of Solar Active Region 2192
• Radiative Hydrodynamic Models of Optical and Ultraviolet Emission from M Dwarf Flares
• The Radiated Energy Budget of Chromospheric Plasma in a Major Solar Flare Deduced From Multi-Wavelength Observations
• The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares I: Modeling the Brightest NUV Footpoints in the X1 Solar Flare of 2014 March 29
• New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
• On the origin of 3 seismic sources in the proton-rich flare of October 28, 2003
• Propagation of Alfvénic Waves From Corona to Chromosphere and Consequences for Solar Flares
• Hydrogen Balmer Continuum in Solar Flares Detected by the Interface Region Imaging Spectrograph (IRIS)
• Hydrogen Balmer Line Broadening in Solar and Stellar Flares
• A Very Bright, Very Hot, and Very Long Flaring Event from the M Dwarf Binary System DG CVn
• Fine structure of flare ribbons and evolution of electric currents
• The Power-Law Distribution of Flare Kernels and Fractal Current Sheets in a Solar Flare
• Kepler Flares IV: A Comprehensive Analysis of the Activity of the dM4e Star GJ 1243
• Parameterizations of Chromospheric Condensations in dG and dMe Model Flare Atmospheres
• On the nature of off-limb flare continuum sources detected by SDO/HMI
• Characteristics that Produce White-Light Enhancements in Solar Flares Observed by Hinode/SOT
• Optical Spectral Observations of a Flickering White-Light Kernel in a C1 Solar Flare
• On the origin of the flare emission in IRIS' SJI 2832 filter: Balmer continuum or spectral lines?
• Suppression of Hydrogen Emission in an X-class White-light Solar Flare

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