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 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. Access through your institution Buy or subscribe This is a preview of subscription content, access

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ADS  Google Scholar  Download references 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. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Space Weather Research Laboratory, New Jersey Institute of Technology, University Heights, Newark,

07102-1982, New Jersey, USA. Haimin Wang, Chang Liu, Yan Xu, Ju Jing, Na Deng & Nengyi Huang * Big Bear Solar Observatory, New Jersey Institute of Technology, 40386 North Shore Lane, Big

Bear City, 92314-9672, California, USA. Haimin Wang, Chang Liu, Kwangsu Ahn, Yan Xu, Ju Jing, Na Deng, Nengyi Huang & Wenda Cao * Center for Solar-Terrestrial Research, New Jersey

Institute of Technology, University Heights, Newark, 07102-1982, New Jersey, USA. Haimin Wang, Chang Liu, Yan Xu, Ju Jing, Na Deng, Nengyi Huang, Gregory D. Fleishman, Dale E. Gary & 

Wenda Cao * Department of Geophysics and Planetary Sciences, CAS Key Laboratory of Geospace Environment, University of Science and Technology of China, Hefei, 230026, China. Rui Liu *

Collaborative Innovation Center of Astronautical Science and Technology, Hefei, China. Rui Liu * Institute for Space-Earth Environmental Research, Nagoya University, Furo-cho, Chikusa-ku,

Nagoya, 464-8601, Japan. Kanya Kusano Authors * Haimin Wang View author publications You can also search for this author inPubMed Google Scholar * Chang Liu View author publications You can

also search for this author inPubMed Google Scholar * Kwangsu Ahn View author publications You can also search for this author inPubMed Google Scholar * Yan Xu View author publications You

can also search for this author inPubMed Google Scholar * Ju Jing View author publications You can also search for this author inPubMed Google Scholar * Na Deng View author publications You

can also search for this author inPubMed Google Scholar * Nengyi Huang View author publications You can also search for this author inPubMed Google Scholar * Rui Liu View author publications

You can also search for this author inPubMed Google Scholar * Kanya Kusano View author publications You can also search for this author inPubMed Google Scholar * Gregory D. Fleishman View

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Cao View author publications You can also search for this author inPubMed Google Scholar 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. ETHICS DECLARATIONS COMPETING INTERESTS 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) RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE 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 Download citation * Received: 11

October 2016 * Accepted: 22 February 2017 * Published: 27 March 2017 * DOI: https://doi.org/10.1038/s41550-017-0085 SHARE THIS ARTICLE Anyone you share the following link with will be able

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