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Abstract Between 2017 and 2024, the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory has observed numerous white-light solar flares (WLFs). HMI spectropolarimetric observations of certain WLFs, in particular the X9.3 flare of 2017 September 6, reveal one or more locations within the umbra or along the umbra/penumbra boundary of the flaring active region where the FeI6173 Å line briefly goes into full emission, indicating significant heating of the photosphere and lower chromosphere. For five flares featuring FeI6173 Å line-core emission, we perform spectropolarimetric analysis using HMI 90 s cadence Stokes data. For all investigated flares, line-core emission is observed to last for a single 90 s frame and is either concurrent with or followed by an increase in the line continuum intensity lasting one to two frames (90–180 s). Additionally, permanent changes to the StokesQ,U, and/orVprofiles were observed, indicating long-lasting nontransient changes to the photospheric magnetic field. These emissions coincided with local maxima in hard X-ray emission observed by Konus-Wind, as well as local maxima in the time derivative of soft X-ray emission observed by GOES 16-18. Comparison of the FeI6173 Å line profile synthesis for the ad hoc heating of the initial empirical VAL-S umbra model and quiescent-Sun (VAL-C-like) model indicates that the FeI6173 Å line emission in the white-light flare kernels could be explained by the strong heating of initially cool photospheric regions.
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Observations of Extremely Strong Magnetic Fields in Active Region NOAA 12673 Using GST Magnetic Field Measurement
Abstract We present a detailed study of very strong magnetic fields in the NOAA Active Region (AR) 12673, which was the most flare productive AR in solar cycle 24. It produced four X-class flares including the X9.3 flare on 2017 September 6 and the X8.2 limb event on September 10. Our analysis is based on direct measurements of full Zeeman splitting of the Fei1564.85 nm line using all Stokes I, Q, U, and V profiles. This approach allowed us to obtain reliable estimates of the magnitude of magnetic fields independent of the filling factor and atmosphere models. Thus, the strongest fields up to 5.5 kG were found in a light bridge (LB) of a spot, while in the dark umbra magnetic fields did not exceed 4 kG. In the case of the LB, the magnitude of the magnetic field is not related to the underlying continuum intensity, while in the case of umbral fields we observed a well-known anticorrelation between the continuum intensity and the field magnitude. In this study, the LB was cospatial with a polarity inversion line ofδ-sunspot, and we speculate that the 5.5 kG strong horizontal fields may be associated with a compact twisted flux rope at or near the photosphere. A comparison of the depth of the Zeemanπandσcomponents showed that in the LB magnetic fields are, on average, more horizontal than those in the dark umbra.
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- PAR ID:
- 10364289
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 928
- Issue:
- 1
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 41
- Size(s):
- Article No. 41
- Sponsoring Org:
- National Science Foundation
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