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

Journal
Date Issued
2019
Author(s)
Adam F. Kowalski
•
Elizabeth Butler
•
Adrian N. Daw
•
Lyndsay Fletcher
•
Joel C. Allred
•
Bart De Pontieu
•
Graham S. Kerr
•
DOI
Abstract
The GOES X1 flare SOL2014-10-25T17:08:00 was a three-ribbon solar flare observed with the Interface Region Imaging Spectrograph (IRIS) in the near-UV (NUV) and far-UV. One of the flare ribbons crossed a sunspot umbra, producing a dramatic, ∼1000% increase in the NUV continuum radiation. We comprehensively analyze the UV 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....
Volume
878
Issue
2
Start page
135
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2019ApJ...878..135K
Rights
open.access
File(s)
Loading...
Thumbnail Image
Name

Kowalski_2019_ApJ_878_135.pdf

Description
Pdf editoriale
Size

4.67 MB

Format

Adobe PDF

Checksum (MD5)

021a12b266ebe9aebf807e318d39f12b