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High-resolution observations of flare precursors in the low solar atmosphere

Abstract

Solar flares are generally believed to be powered by free magnetic energy stored in the corona1, but the build up of coronal energy alone may be insufficient to trigger the flare to occur2. The flare onset mechanism is a critical but poorly understood problem, insights into which could be gained from small-scale energy releases known as precursors. These precursors are observed as small pre-flare brightenings in various wavelengths313 and also from certain small-scale magnetic configurations such as opposite-polarity fluxes1416, where the magnetic orientation of small bipoles is opposite to that of the ambient main polarities. However, high-resolution observations of flare precursors together with the associated photospheric magnetic field dynamics are lacking. Here we study precursors of a flare using the unprecedented spatiotemporal resolution of the 1.6-m New Solar Telescope, complemented by new microwave data. Two episodes of precursor brightenings are initiated at a small-scale magnetic channel1720 (a form of opposite-polarity flux) with multiple polarity inversions and enhanced magnetic fluxes and currents, lying near the footpoints of sheared magnetic loops. Microwave spectra corroborate that these precursor emissions originate in the atmosphere. These results provide evidence of low-atmospheric small-scale energy release, possibly linked to the onset of the main flare.

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Figure 1: Precursor brightenings.
Figure 2: Magnetic field structure and evolution.
Figure 3: Properties of magnetic channel region.
Figure 4: Microwave emission.

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Acknowledgements

We thank the BBSO, EOVSA, SDO, RHESSI, GOES and Hinode teams for obtaining the data. This work was supported by NASA under grants NNX13AF76G, NNX13AG13G, NNX14AC12G, NNX14AC87G, NNX16AL67G and NNX16AF72G, and by the NSF under grants AGS 1250374, 1262772, 1250818, 1348513, 1408703 and 1539791. R.L. acknowledges support from the Thousand Young Talents Program of China and NSFC 41474151. K.K. acknowledges support from MEXT/JSPS KAKENHI 15H05814. The BBSO operation is supported by NJIT, US NSF AGS 1250818 and NASA NNX13AG14G grants, and partly supported by the Korea Astronomy and Space Science Institute and Seoul National University and by the Chinese Academy of Science’s strategic priority research programme, Grant No. XDB09000000.

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Authors

Contributions

H.W. initiated the idea and carried out the data processing, analysis, interpretation and manuscript writing. C.L. contributed to the azimuth disambiguation of NIRIS data, data analysis and interpretation, and manuscript revision. K.A. developed tools for NIRIS data calibration, polarization inversion, and processed the NIRIS data. Y.X. was the Principal Investigator for this BBSO/NST observation run and contributed to the data processing. J.J. and N.D. contributed to the data analysis. N.H. contributed to this NST observation run. R.L. contributed to the NLFFF modelling and result interpretation. K.K. contributed to the interpretation of observations. G.D.F. and D.E.G. carried out the microwave data analysis and modelling. W.C. developed instruments at BBSO. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Haimin Wang or Wenda Cao.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Figures 1–4 (PDF 1558 kb)

Supplementary Video 1

Time sequence of BBSO/NST H+ 0.6 Å images. (MP4 2131 kb)

Supplementary Video 2

Time sequence of BBSO/NST NIRIS photospheric vertical magnetic field images. (MP4 2160 kb)

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Wang, H., Liu, C., Ahn, K. et al. High-resolution observations of flare precursors in the low solar atmosphere. Nat Astron 1, 0085 (2017). https://doi.org/10.1038/s41550-017-0085

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