Mysterious polarization signal confirmed in a H-alpha solar flare
We performed statistical and event studies of linear polarization in the H-alpha (Hα) line during solar flares [1]. The statistical study revealed that, among 71 Hα flares analyzed, including 64 GOES flares, only one event shows significant linear polarization signals. Such an infrequent occurrence of significant linear polarization in solar flares is consistent with the result by Bianda et al. (2005), who studied 30 flares and found no polarization signals.
Figure displays images of the Hα flare showing significant linear polarization signals. In the event showing the significant polarization, the maximum degree of linear polarization was 1.16 ± 0.06 percent, and the average direction of the polarization deviated by -142.5 ± 6.0 degrees from the solar north. The observed polarization degrees and the directions are consistent with the preceding reports. These strong linear polarization signals did not appear at major flare ribbons, nor did they correlate with either hard or soft X-ray emissions temporally or spatially. Instead they appeared at a minor flare kernel, which corresponds to one of the footpoints of a coronal loop. The active region caused coronal dimming after the soft X-ray peak.
The observed flare shows no direct evidence that the linear polarization is produced by high energy particles, which are often considered to generate the polarization. On the other hand, our study suggests the possibility that coronal mass ejections, which have been often observed in flares showing linear polarization signals, play an important role for exciting linear polarization at Hα flare kernels.
[1] Kawate1 and Hanaoka2, 2019, The Astrophysical Journal, 872, 74, DOI:10.3847/1538-4357/aafe0f (1Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency; 2National Astronomical Observatory of Japan, National Institutes of Natural Science)
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Figure. Maps of Hα intensity, linear polarization (U/I), MDI continuum intensity, and EIT 195 Å intensity for the Hα flare showing significant linear polarization signals. In each map, contours exhibit Hα intensity levels. A green arrow in the U/I map indicates the Hα ribbon with a strong polarization signal. A blue arrow in the EIT image shows the direction of polarization, which corresponds to the tangential direction of the coronal loop structure at the footpoint. MDI and EIT data are provided by courtesy of SOHO consortium and SOHO is a project of international cooperation between ESA and NASA. |
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