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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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This content will become publicly available on November 24, 2026
Observations of Weak Magnetic Fields in the Solar Chromosphere with Two He I Lines
Abstract Solar magnetic fields are responsible for solar coronal mass ejections and other eruptive phenomena that govern space weather. Today, most of our knowledge about the solar magnetic field topologies was derived from the measurements of the magnetic field of the solar photosphere. The solar chromosphere is dilute, and associated magnetic fields are weak, which makes them difficult to measure. To address this challenging issue, we propose the use of the HeID35876 Å line, together with the HeI10830 Å line, to measure the off-limb weak and moderate chromospheric magnetic fields in the Hanle effect regime. We show that the current approach of using the polarization amplitude or degree cannot reliably retrieve the magnetic fields. We demonstrated that by using the Stokes linear polarization profiles simultaneously with the 5876 and 10830 Å lines, without the use of the circular polarization, chromospheric magnetic fields as low as and down to a few gauss can be measured easily and reliably with today’s instruments. We further demonstrate the potential to measure the weak chromospheric magnetic fields with the Stokes linear polarization profiles by using the He 5876 Å line alone, which has not been fully investigated yet. This work will provide a new technique for future daily synoptic observations for the weak magnetic fields in the solar chromosphere for both the active and quiescent regions, which are still difficult to measure.
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- PAR ID:
- 10653046
- Publisher / Repository:
- Research in Astronomy and Astrophysics
- Date Published:
- Journal Name:
- Research in Astronomy and Astrophysics
- Volume:
- 26
- Issue:
- 1
- ISSN:
- 1674-4527
- Page Range / eLocation ID:
- 015006
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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