Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

The origin of underdense plasma downflows associated with magnetic reconnection in solar flares

Abstract

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.

This is a preview of subscription content, access via your institution

Access options

Buy article

Get time limited or full article access on ReadCube.

$32.00

All prices are NET prices.

Fig. 1: Observations and 3D modelling of the energy-release region of a solar flare.
Fig. 2: Detailed face-on view of the reconnection current sheet and the turbulent interface region in the 3D MHD model.
Fig. 3: Development of finger-like dynamic structures in the interface region underneath the current sheet.
Fig. 4: Schematic of the development of RTI/RMI instabilities in the turbulent interface region beneath the reconnection current sheet.

Data availability

The SDO/AIA data are publicly available and obtained using the SunPy module Fido.

Code availability

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/.

References

  1. Forbes, T. G. & Acton, L. W. Reconnection and field line shrinkage in solar flares. Astrophys. J. 459, 330–341 (1996).

    ADS  Google Scholar 

  2. Haggerty, C. C. et al. The reduction of magnetic reconnection outflow jets to sub-Alfvénic speeds. Phys. Plasmas 25, 102120 (2018).

    ADS  Google Scholar 

  3. McKenzie, D. E. & Hudson, H. S. X-ray observations of motions and structure above a solar flare arcade. Astrophys. J. 519, L93–L96 (1999).

    ADS  Google Scholar 

  4. Savage, S. L., McKenzie, D. E. & Reeves, K. K. Re-interpretation of supra-arcade downflows in solar flares. Astrophys. J. Lett. 747, L40 (2012).

    ADS  Google Scholar 

  5. Savage, S. L. & McKenzie, D. E. Quantitative examination of a large sample of supra-arcade downflows in eruptive solar flares. Astrophys. J. 730, 98 (2011).

    ADS  Google Scholar 

  6. Innes, D. E., Guo, L. J., Bhattacharjee, A., Huang, Y. M. & Schmit, D. Observations of supra-arcade fans: instabilities at the head of reconnection jets. Astrophys. J. 796, 27 (2014).

    ADS  Google Scholar 

  7. Innes, D. E., McKenzie, D. E. & Wang, T. Observations of 1,000 km s−1 Doppler shifts in 107 K solar flare supra-arcade. Sol. Phys. 217, 267–279 (2003).

    ADS  Google Scholar 

  8. Lin, J. et al. Direct observations of the magnetic reconnection site of an eruption on 2003 November 18. Astrophys. J. 622, 1251–1264 (2005).

    ADS  Google Scholar 

  9. Liu, W., Chen, Q. & Petrosian, V. Plasmoid ejections and loop contractions in an eruptive M7.7 solar flare: evidence of particle acceleration and heating in magnetic reconnection outflows. Astrophys. J. 767, 168 (2013).

    ADS  Google Scholar 

  10. Warren, H. P. et al. Spectroscopic observations of current sheet formation and evolution. Astrophys. J. 854, 122 (2018).

    ADS  Google Scholar 

  11. Cassak, P. A. et al. On the cause of supra-arcade downflows in solar flares. Astrophys. J. 775, L14 (2013).

    ADS  Google Scholar 

  12. Cécere, M., Zurbriggen, E., Costa, A. & Schneiter, M. 3D MHD simulation of flare supra-arcade downflows in a turbulent current sheet medium. Astrophys. J. 807, 6 (2015).

    ADS  Google Scholar 

  13. Guo, L. J., Huang, Y. M., Bhattacharjee, A. & Innes, D. E. Rayleigh–Taylor type instabilities in the reconnection exhaust jet as a mechanism for supra-arcade downflows in the Sun. Astrophys. J. 796, L29 (2014).

    ADS  Google Scholar 

  14. Reeves, K. K., Freed, M. S., McKenzie, D. E. & Savage, S. L. An exploration of heating mechanisms in a supra-arcade plasma sheet formed after a coronal mass ejection. Astrophys. J. 836, 55 (2017).

    ADS  Google Scholar 

  15. Longcope, D., Unverferth, J., Klein, C., McCarthy, M. & Priest, E. Evidence for downflows in the narrow plasma sheet of 2017 September 10 and their significance for flare reconnection. Astrophys. J. 868, 148 (2018).

    ADS  Google Scholar 

  16. Masuda, S., Kosugi, T., Hara, H., Tsuneta, S. & Ogawara, Y. A loop-top hard X-ray source in a compact solar flare as evidence for magnetic reconnection. Nature 371, 495–497 (1994).

    ADS  Google Scholar 

  17. Krucker, S. et al. Measurements of the coronal acceleration region of a solar flare. Astrophys. J. 714, 1108–1119 (2010).

    ADS  Google Scholar 

  18. Chen, B. et al. Measurement of magnetic field and relativistic electrons along a solar flare current sheet. Nat. Astron. 4, 1140–1147 (2020).

    ADS  Google Scholar 

  19. Fleishman, G. D. et al. Decay of the coronal magnetic field can release sufficient energy to power a solar flare. Science 367, 278–280 (2020).

    ADS  MathSciNet  MATH  Google Scholar 

  20. Chen, B. et al. Particle acceleration by a solar flare termination shock. Science 350, 1238–1242 (2015).

    ADS  Google Scholar 

  21. Takasao, S., Matsumoto, T., Nakamura, N. & Shibata, K. Magnetohydrodynamic shocks in and above post-flare loops: two-dimensional simulation and a simplified model. Astrophys. J. 805, 135 (2015).

    ADS  Google Scholar 

  22. Somov, B. V. & Kosugi, T. Collisionless reconnection and high-energy particle acceleration in solar flares. Astrophys. J. 485, 859–868 (1997).

    ADS  Google Scholar 

  23. Reeves, K. K. et al. Hot plasma flows and oscillations in the loop-top region during the 2017 September 10 X8.2 solar flare. Astrophys. J. 905, 165 (2020).

    ADS  Google Scholar 

  24. Miles, A. R. The blast-wave-driven instability as a vehicle for understanding supernova explosion structure. Astrophys. J. 696, 498–514 (2009).

    ADS  Google Scholar 

  25. Warren, J. S. et al. Cosmic-ray acceleration at the forward shock in Tycho’s supernova remnant: evidence from Chandra X-ray observations. Astrophys. J. 634, 376–389 (2005).

    ADS  Google Scholar 

  26. Hanneman, W. J. & Reeves, K. K. Thermal structure of current sheets and supra-arcade downflows in the solar corona. Astrophys. J. 786, 95 (2014).

    ADS  Google Scholar 

  27. Savage, S. L., McKenzie, D. E., Reeves, K. K., Forbes, T. G. & Longcope, D. W. Reconnection outflows and current sheet observed with Hinode/XRT in the 2008 April 9 ‘Cartwheel CME’ flare. Astrophys. J. 722, 329–342 (2010).

    ADS  Google Scholar 

  28. Yu, S. et al. Magnetic reconnection during the post-impulsive phase of a long-duration solar flare: bidirectional outflows as a cause of microwave and X-ray bursts. Astrophys. J. 900, 17 (2020).

    ADS  Google Scholar 

  29. Su, Y. et al. Imaging coronal magnetic-field reconnection in a solar flare. Nat. Phys. 9, 489–493 (2013).

    Google Scholar 

  30. Stone, J. M., Gardiner, T. A., Teuben, P., Hawley, J. F. & Simon, J. B. Athena: a new code for astrophysical MHD. Astrophys. J. Supp. 178, 137–177 (2008).

    ADS  Google Scholar 

  31. Lemen, J. R. et al. The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO). Sol. Phys. 275, 17–40 (2012).

    ADS  Google Scholar 

  32. O’Dwyer, B., Del Zanna, G., Mason, H. E., Weber, M. A. & Tripathi, D. SDO/AIA response to coronal hole, quiet Sun, active region, and flare plasma. Astron. Astrophys. 521, A21 (2010).

    Google Scholar 

  33. Shen, C., Lin, J. & Murphy, N. Numerical experiments on fine structure within reconnecting current sheets in solar flares. Astrophys. J. 737, 14 (2011).

    ADS  Google Scholar 

  34. Shen, C., Kong, X., Guo, F., Raymond, J. C. & Chen, B. The dynamical behavior of reconnection-driven termination shocks in solar flares: magnetohydrodynamic simulations. Astrophys. J. 869, 116 (2018).

    ADS  Google Scholar 

  35. Meyer, C. D., Balsara, D. S. & Aslam, T. D. A second-order accurate Super TimeStepping formulation for anisotropic thermal conduction. Mon. Not. R. Astron. Soc. 422, 2102–2115 (2012).

    ADS  Google Scholar 

  36. Klimchuk, J. A., Patsourakos, S. & Cargill, P. J. Highly efficient modeling of dynamic coronal loops. Astrophys. J. 682, 1351–1362 (2008).

    ADS  Google Scholar 

  37. Yokoyama, T. & Shibata, K. Magnetohydrodynamic simulation of a solar flare with chromospheric evaporation effect based on the magnetic reconnection model. Astrophys. J. 549, 1160–1174 (2001).

    ADS  Google Scholar 

  38. Kopp, R. & Pneuman, G. Magnetic reconnection in the corona and the loop prominence phenomenon. Sol. Phys. 50, 85–98 (1976).

    ADS  Google Scholar 

  39. Boerner, P. et al. Initial calibration of the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO). Sol. Phys. 275, 41–66 (2012).

    ADS  Google Scholar 

  40. Dere, K. P., Del Zanna, G., Young, P. R., Landi, E. & Sutherland, R. S. CHIANTI—an atomic database for emission lines. XV. Version 9, improvements for the X-ray satellite lines. Astrophys. J. Supp. 241, 22 (2019).

    ADS  Google Scholar 

  41. Feldman, U. Elemental abundances in the upper solar atmosphere. Phys. Scr. 46, 202–220 (1992).

    ADS  Google Scholar 

  42. Priest, E. & Forbes, T. Magnetic Reconnection: MHD Theory and Applications Ch. 4 (Cambridge Univ. Press, 2000).

  43. Petschek, H. E. Magnetic Field Annihilation Vol. 50 (NASA Special Publication,1964).

  44. Forbes, T. G. & Priest, E. R. A comparison of analytical and numerical models for steadily driven magnetic reconnection. Rev. Geophys. 25, 1583–1607 (1987).

    ADS  Google Scholar 

  45. Yokoyama, T. & Shibata, K. What is the condition for fast magnetic reconnection? Astrophys. J. Lett. 436, L197–L200 (1994).

    ADS  Google Scholar 

  46. Strauss, H. R. Turbulent reconnection. Astrophys. J. 326, 412–417 (1988).

    ADS  Google Scholar 

  47. Lazarian, A. et al. Turbulence, magnetic reconnection in turbulent fluids and energetic particle acceleration. Space Sci. Rev. 173, 557–622 (2012).

    ADS  Google Scholar 

  48. Lazarian, A. et al. 3D turbulent reconnection: theory, tests, and astrophysical implications. Phys. Plasmas 27, 012305 (2020).

    ADS  Google Scholar 

  49. Loureiro, N. F., Schekochihin, A. A. & Cowley, S. C. Instability of current sheets and formation of plasmoid chains. Phys. Plasmas 14, 100703 (2007).

    ADS  Google Scholar 

  50. Bhattacharjee, A., Huang, Y.-M., Yang, H. & Rogers, B. Fast reconnection in high-Lundquist-number plasmas due to the plasmoid instability. Phys. Plasmas 16, 112102 (2009).

    ADS  Google Scholar 

  51. Ni, L. et al. Linear plasmoid instability of thin current sheets with shear flow. Phys. Plasmas 17, 052109 (2010).

    ADS  Google Scholar 

  52. Mei, Z. et al. Numerical experiments on magnetic reconnection in solar flare and coronal mass ejection current sheets. Mon. Not. R. Astron. Soc. 425, 2824–2839 (2012).

    ADS  Google Scholar 

  53. Ye, J., Shen, C., Raymond, J. C., Lin, J. & Ziegler, U. Numerical study of the cascading energy conversion of the reconnection current sheet in solar eruptions. Mon. Not. R. Astron. Soc. 482, 588–605 (2019).

    ADS  Google Scholar 

  54. Ji, H. & Daughton, W. Phase diagram for magnetic reconnection in heliophysical, astrophysical, and laboratory plasmas. Phys. Plasmas 18, 111207 (2011).

    ADS  Google Scholar 

  55. Cassak, P. A., Liu, Y. H. & Shay, M. A. A review of the 0.1 reconnection rate problem. J. Plasma Phys. 83, 715830501 (2017).

    Google Scholar 

  56. Huang, Y.-M. & Bhattacharjee, A. Turbulent magnetohydrodynamic reconnection mediated by the plasmoid instability. Astrophys. J. 818, 20 (2016).

    ADS  Google Scholar 

  57. Yang, L. et al. Fast magnetic reconnection with turbulence in high Lundquist number limit. Astrophys. J. Lett. 901, L22 (2020).

    ADS  Google Scholar 

  58. Zhou, Y. Rayleigh–Taylor and Richtmyer–Meshkov instability induced flow, turbulence, and mixing. I. Phys. Rep. 720, 1–136 (2017).

    ADS  MathSciNet  MATH  Google Scholar 

  59. Alon, U., Hecht, J., Ofer, D. & Shvarts, D. Power laws and similarity of Rayleigh–Taylor and Richtmyer–Meshkov mixing fronts at all density ratios. Phys. Rev. Lett. 74, 534–537 (1995).

    ADS  Google Scholar 

  60. McKenzie, D. E. Turbulent dynamics in solar flare sheet structures measured with local correlation tracking. Astrophys. J. 766, 39 (2013).

    ADS  Google Scholar 

  61. Samanta, T. et al. Plasma heating induced by tadpole-like downflows in the flaring solar corona. Innovation 2, 100083 (2021).

    Google Scholar 

  62. Aschenbach, B., Egger, R. & Trümper, J. Discovery of explosion fragments outside the Vela supernova remnant shock-wave boundary. Nature 373, 587–590 (1995).

    ADS  Google Scholar 

  63. Balick, B. & Frank, A. Shapes and shaping of planetary nebulae. Annu. Rev. Astron. Astrophys. 40, 439–486 (2002).

    ADS  Google Scholar 

  64. Attal, N. & Ramaprabhu, P. Numerical investigation of a single-mode chemically reacting Richtmyer–Meshkov instability. Shock Waves 25, 307–328 (2015).

    ADS  Google Scholar 

  65. Chen, F., Xu, A. & Zhang, G. Collaboration and competition between Richtmyer–Meshkov instability and Rayleigh–Taylor instability. Phys. Fluids 30, 102105 (2018).

    ADS  Google Scholar 

  66. Wheatley, V., Gehre, R. M., Samtaney, R. & Pullin, D. I. The magnetohydrodynamic Richtmyer–Meshkov instability: the oblique field case. In 29th International Symposium on Shock Waves (eds. Riccardo, B & Devesh, R) Vol. 2 1107-1112 (Springer International Publishing, 2015).

Download references

Acknowledgements

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.

Author information

Authors and Affiliations

Authors

Contributions

C.S. performed the MHD simulations and analysed the results. B.C. and K.R.R. proposed the study and contributed to the modelling setup and results analysis. S.Y., V.P. and X.X. contributed to EUV data collection, analysis and visualization. C.S. and B.C. led the manuscript writing, and all authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Chengcai Shen.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Astronomy thanks the anonymous reviewers for their contribution to the peer review of this work.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary information.

Supplementary Video 1

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.

Supplementary Video 2

Evolution of the flare fan and SADs observed by SDO/AIA in its 131 Å EUV filter band on 2015 June 18.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41550-021-01570-2

This article is cited by

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing