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 wavelengths3–13 and also from certain small-scale magnetic configurations such as opposite-polarity fluxes14–16, 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 channel17–20 (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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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.
The authors declare no competing financial interests.
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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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