Magnetic reconnection is a universal process that powers explosive energy-release events such as solar flares, geomagnetic substorms and some astrophysical jets. A characteristic feature of magnetic reconnection is the production of fast reconnection outflow jets near the plasma Alfvén speeds1,2. In eruptive solar flares, dark finger-shaped plasma downflows moving toward the flare arcade have been commonly regarded as the principal observational evidence for such reconnection-driven outflows3,4. However, they often show a speed much slower than that expected in reconnection theories5,6, challenging the reconnection-driven energy-release scenario in standard flare models. Here we present a three-dimensional magnetohydrodynamics model of solar flares. By comparing the model predictions with the observed plasma downflow features, we conclude that these dark downflows are self-organized structures formed in a turbulent interface region below the flare termination shock where the outflows meet the flare arcade, a phenomenon analogous to the formation of similar structures in supernova remnants. This interface region hosts a myriad of turbulent flows, electron currents and shocks, crucial for flare energy release and particle acceleration.
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The SDO/AIA data are publicly available and obtained using the SunPy module Fido.
The MHD code is accessible at https://princetonuniversity.github.io/Athena-Cversion/. The AIA data are analysed using the SunPy (https://github.com/sunpy) and AIApy packages (https://pypi.org/project/aiapy). The SolarSoft (SSW) package is obtained from https://www.lmsal.com/solarsoft/ssw. The Chianti atomic data are obtained through https://www.chiantidatabase.org/.
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The authors thank L. Guo for the help on the modelling setup and J. Raymond, N. Murphy and J. Lin for helpful discussions. The AIA is an instrument on SDO, a National Aeronautics and Space Administration mission. CHIANTI is a collaborative project involving George Mason University (USA), the University of Michigan (USA) and the University of Cambridge (UK). The computations in this paper were conducted on the Smithsonian High Performance Cluster, Smithsonian Institution (https://doi.org/10.25572/SIHPC). C.S. and K.R.R. are supported by National Science Foundation grants AST-1735525, AGS-1723313 and AGS-1723425 to Smithsonian Astrophysical Observatory. B.C. and S.Y. are supported by National Science Foundation grants AGS-1654382, AGS-1723436 and AST-1735405 to New Jersey Institute of Technology. V.P. acknowledges support from National Science Foundation Solar Heliospheric and INterplanetary Environment grant AGS-1723409. X.X. is supported by the Chinese Academy of Sciences grants XDA17040507 and QYZDJ-SSWSLH012, National Natural Science Foundation of China grant 11933009, Yunnan Province grant 2018HC023 and the scholarship granted by the China Scholarship Council under file No. 201904910573.
The authors declare no competing interests.
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Temporal evolution of the magnetic field and density in the centre plane (x = 0) in Case A. The perturbations appear at the density interface and gradually develop to finger-like downflows with low density.
Evolution of the flare fan and SADs observed by SDO/AIA in its 131 Å EUV filter band on 2015 June 18.
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Shen, C., Chen, B., Reeves, K.K. et al. The origin of underdense plasma downflows associated with magnetic reconnection in solar flares. Nat Astron 6, 317–324 (2022). https://doi.org/10.1038/s41550-021-01570-2
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