Spectral Evidence for Heating at Large Column Mass in Umbral Solar Flare Kernels. I. IRIS Near-UV Spectra of the X1 Solar Flare of 2014 October 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, ∼ 1000% increase in the nearultraviolet (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 λ2814.45 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 λ2829.91 (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.adam.f.kowalski@colorado.edu 1 Various loose definitions of a "white-light flare" exist, including a flare that could be detected by the eye (which is broadband). In dMe stars, U -band flares are certainly considered white-light flares even though our eye is not sensitive to these wavelengths, and flares detected in a narrow passband of SDO/HMI are often called white-light flares, assuming that continuum radiation is the source of the HMI increase. Generally, a white-light flare is a flare that produces a change in continuum radiation that could be detected in the Johnson U and/or V bands.

The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. II. Hydrogen Broadening Predictions for Solar Flare Observations with the Daniel K. Inouye Solar Telescope

“…A line-to-continuum ratio is an alternative diagnostic that can be calculated from spectra with a narrow wavelength range (Silverberg et al 2016;Kowalski et al 2019b). The motivation is similar to the line-to-continuum ratio of Fe IIλ2814 to the λ ≈ 2826 Å continuum radiation in IRIS/NUV solar flare spectra (Kowalski et al 2019a). Here we use the wavelength-integrated, continuum-subtracted intensity of Hγ divided by the xix pre-flare-subtracted continuum intensity at λ = 4170 Å.…”

“…For proper modeling of the resonance lines of Mg ii and Ca ii, the partial frequency redistribution of opacity in dynamic atmospheres must also be considered (Hawley et al 2007;Rubio da Costa & Kleint 2017;Kerr et al 2019a,b;Zhu et al 2019). In the model of Paper i (see also Kowalski et al 2019a), the variation in the red wing intensity relative to the rest-wavelength intensity in Fe ii λ2814, Fe ii λ2832, and Mg ii λ2791.6 was argued to vary with the optical depth of the line. As hydrogen lines are much more optically thick than these lines, they are expected to be formed over a narrower height range.…”

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

“…Because dMe flares are rarely observed with both high cadence and high resolving power (and never with any direct spatial resolution), the red-shifted satellite components that are expected from extreme (and possibly unrealistic; Krucker et al, 2011) electron-beam fluxes are difficult to validate using only stellar data. A line-to-continuum ratio in IRIS NUV spectra was recently investigated to better diagnose the amount of heating in the low chromosphere or upper photosphere in solar flares (Kowalski et al, 2019a), demonstrating the relative heating rate of these lower layers to be significant relative to that of the upper chromosphere. Other sources of evidence have previously hinted at significant heating below the upper chromosphere (Martínez Oliveros et al, 2012;Warren, 2014); resolving this issue on the Sun would provide important clues for modeling the origin of 10,000 K blackbody-like component that clearly dominates the energetics in some dMe flares.…”

A multi-diagnostic spectral analysis of penumbral microjets

“…This line lies between the Mg ii triplet blend and the Mg ii k line and was recently identified as a Fe ii line in Tian et al (2015). The identification of this line is uncertain, however (see Kowalski et al 2019), and we limit ourselves to reporting the observation that this line is typically in emission whenever the wings of the triplet blend are also in emission.…”