Abstract
Magnetic-field reconnection is believed to play a fundamental role in magnetized plasma systems throughout the Universe1, including planetary magnetospheres, magnetars and accretion disks around black holes. This letter presents extreme ultraviolet and X-ray observations of a solar flare showing magnetic reconnection with a level of clarity not previously achieved. The multi-wavelength extreme ultraviolet observations from SDO/AIA show inflowing cool loops and newly formed, outflowing hot loops, as predicted. RHESSI X-ray spectra and images simultaneously show the appearance of plasma heated to >10 MK at the expected locations. These two data sets provide solid visual evidence of magnetic reconnection producing a solar flare, validating the basic physical mechanism of popular flare models. However, new features are also observed that need to be included in reconnection and flare studies, such as three-dimensional non-uniform, non-steady and asymmetric evolution.
Similar content being viewed by others
Main
The early concept of magnetic reconnection was proposed in the 1940s (ref. 1) to explain energy release in solar flares, the most powerful explosive phenomena in the solar system. The reconnection process reconfigures the field topology and converts magnetic energy to thermal energy, mass motions and particle acceleration. The theories and related numerical simulations, especially three-dimensional modelling, are still subjects of extensive research to obtain a full understanding of the process under different conditions. Meanwhile, observational studies have made progress in finding evidence of reconnection and deriving its physical properties to constrain and improve the theories.
In situ measurements of the magnetic field, plasma parameters and particle distributions have shown the existence of magnetic reconnection in laboratory plasmas2,3, fusion facilities and magnetospheres of planets4,5. Such in situ measurements are still not possible in the extremely hot solar atmosphere. Instead, observations are obtained through remote sensing of emissions across the entire electromagnetic spectrum from radio to X-rays and gamma rays. However, in the corona6 the magnetic field pressure dominates the plasma pressure (low plasma beta) and the magnetic flux is frozen into the highly conductive plasma. As a result, the emitting plasma trapped in coronal loops outlines the geometry of the magnetic field and their structural changes reflect the changes of the field connectivity (in general). Considerable pieces of evidence for features probably linked to reconnection in solar flares7,8 and coronal mass ejections9 (CMEs) have been obtained so far. These include signatures of plasma inflow/outflow10,11,12,13,14, hot cusp structures15, current sheets16,17,18, fast-mode standing shocks19 and plasmoid ejection20. However, most evidence has been indirect and fragmented. Detailed observations of the complete picture are still missing owing to the highly dynamic flare/CME process and limited observational capabilities.
The launch of the Solar Dynamic Observatory21 (SDO) in 2010 significantly improved this situation. In particular, the Atmospheric Imaging Assembly22 (AIA) has enabled continuous imaging of the full Sun in ten extreme ultraviolet (EUV), ultraviolet and visible channels, with a spatial resolution of ∼ 0.6 arcsec and a cadence of 12 s. Simultaneously, the Ramaty High Energy Solar Spectroscopic Imager23 (RHESSI) is continuing to provide X-ray imaging and spectroscopic diagnostics of the heated plasma and accelerated electrons. The flare of interest was observed close to the southeast limb of the solar disk on 17 August 2011, but no CME was detected. The peak soft X-ray flux (1–8 Å) recorded by the X-ray monitor on a Geostationary Operational Environmental Satellite (GOES) was 2.3×10−6 W m−2, implying a modest flare magnitude of C2.3. The favourable timing, intensity, position and orientation allowed us to observe the most complete evolution yet obtained of magnetic reconfiguration and energy release in an interacting coronal loop arcade.
The clearest visual evidence of the reconnection process comes from the synchronous imaging of cool inflow loops (Fig. 1a,b) and hot outflow loops (Fig. 1c and see also Supplementary Movies 1–3). From ∼ 04:05 UT to 04:28 UT, discrete coronal loops with temperatures from ∼ 0.05 to 2 MK merged and disappeared near a central plane (the red line in Fig. 1b–c), the same location where a hot X-shaped structure (∼ 10 MK, first image in Fig. 1c) gradually formed by ∼ 04:10 UT. The two cusps above and below the X-type neutral point began to separate to form a V-inverted V structure at ∼ 04:15 UT (second image in Fig. 1c). Newly formed hot loops (∼ 10 MK) then appeared in two separated groups. Both flow away from the reconnection region (Fig. 1c), presumably owing to magnetic tension forces.
The bright flare regions on the surface (observed at AIA 1,600 Å, see Fig. 2b and Supplementary Fig. S1) are the footpoints of coronal loops heated by thermal conduction and/or non-thermal electrons. As a result of the reconnection, the magnetic topology probably changed from an arcade of loops connecting footpoints A–B in the southern end and C–D in the northern end to two vertically separated new sets of loops (Fig. 2b). One group connects the lower cusp to regions A and C. This relaxes downward onto the flare arcade. The other group probably connected the higher cusp to the regions B and D. This expands outward, contributing to the loops (magnetic flux rope) building above the arcade.
X-ray emissions from the heated plasma confirm the energy release from a reconnection process in both timing and location. The RHESSI X-ray flux began to increase at 04:06:40 UT, close to the time when the EUV inflow loops visibly started moving together (Fig. 3d). The GOES 1–8 Å flux started to decrease after ∼ 04:29 UT, the time when the last inflow loops (Fig. 3d) were seen disappearing. On the other hand, X-ray images and spectra show heated plasma (up to 17 MK, Fig. 2c) in the reconnected flare loops. The RHESSI coronal source in the 4–10 keV energy range (first and third images of Fig. 2a) indicates the presence of plasma at > 6 MK at/near the reconnection site. After 04:19 UT, a high coronal source appeared in the 10–20 keV images above the higher, relaxing cusp (Fig. 2a and Fig. 4b). This source and the extended source above the flare loops are known as double coronal X-ray sources, first observed in RHESSI images alone for a different flare16. Unlike the famous single Masuda source19, which may be a signature of fast-mode standing shocks below the reconnection site, double coronal sources are thought to be related to heating in both the upper and lower outflow regions. With the support from the EUV images presented here, the double coronal sources become strong evidence for plasma heating following a reconnection process.
The formation of either a single Masuda source or double corona sources remains an open question. In particular, the higher coronal source has been rarely detected and discussed24. The absence of a clear power-law bremsstrahlung component in the RHESSI spectra may indicate a negligible contribution of heating from non-thermal electrons in this case. One mechanism that could explain the double coronal sources is the slow mode magnetosonic shocks during flux retraction, which cause heating and density enhancements in the contracting loops ejected from the reconnection site25.
To quantitatively investigate the inflow and outflow motions evident in the EUV movies, we defined two curves labelled C1 and C2 in Fig. 1c. The resulting time–distance plots for C1 (Fig. 3a) show coronal slices of loops that moved inward (towards the central line) from both sides and disappeared. The apparent inflow velocities increased from ∼ 10 to over 50 km s−1 as the loops approached the point of disappearance. The final inflow velocities range from ∼ 20 to ∼70 km s−1. The outflows are evident in the time–distance plots made for C2 from the 131 Å images (Fig. 3b). Signatures of some superposed fast-moving structures can be seen from 04:08 to 04:22 UT (Fig. 3b and solid cyan lines in Fig. 3d) along the paths of the two separating cusps (the two dashed cyan lines in Fig. 3d). We suggest that these structures are the new loops ejected from the reconnection region, and the two cusps are the subsequently heated thermal sources. The initial outflow velocities (V out) range from ∼ 90 to ∼440 km s−1. If we neglect the projection effect and assume14 the measured outflow velocity is approximately the local Alfvén velocity (V A), we estimate that the reconnection rate, MA = V in/V A≈V in/V out, varies from ∼ 0.05 to 0.5 (Fig. 3e).
The event also revealed other new features of inflows. Some inflow loops originated far from the reconnection site (up to 29,000 km). The inflow loops, visible in different AIA channels, covered a wide range of temperatures from ∼ 0.05 to 2 MK. This means that the plasma across the inflow field had different physical states. Compared with the steady, uniform case, the variable inflowing plasma could result in a variable heating rate and may help explain spikes observed in hard X-ray light curves during the impulsive phase of most flares. The highly curved inflow loops expanded as they approached the reconnection site. Detailed flow velocity maps (Fig. 4b) derived from the Fourier local correlation tracking26 method quantify this expansion. The diverging inflows, together with the signature of piling-up loops (white arrows in Fig. 3 and Supplementary Fig. S2) and the high reconnection rates, are suggestive of flux-pile-up reconnection1 rather than the classic Petschek-like reconnection27. The inflows were apparently asymmetric, assuming that inflow velocity vectors in both inflow regions are in the same plane. For example, the apparent inflow velocities at 04:20–21 UT were ∼50 km s−1 from the south and ∼20 km s−1 from the north. As the reconnection process consumes the same amount of opposite magnetic fluxes from the two inflow regions, one would expect that a weaker field requires a faster inflow speed. Then the region to the south would thus have a weaker field.
The inflow of magnetic loops is not a feature of the standard flare arcade reconnection model, in which loops within a linear arcade reconnect with each other. This inflow13 and the lateral development of the flare arcade are intrinsically three-dimensional phenomena28. Realistic three-dimensional simulations are required to understand the coronal magnetic reconnection and energy release processes observed here.
References
Priest, E. & Forbes, T. Magnetic Reconnection (Cambridge Univ. Press, 2000).
Yamada, M., Kulsrud, R. & Ji, H. T. Magnetic reconnection. Rev. Mod. Phys. 82, 603–664 (2010).
Zhong, J. Y. et al. Modelling loop-top X-ray source and reconnection outflows in solar flares with intense lasers. Nature Phys. 6, 984–987 (2010).
Deng, X. H. & Matsumoto, H. Rapid magnetic reconnection in the Earth’s magnetosphere mediated by whistler waves. Nature 410, 557–560 (2001).
Øieroset, M., Phan, T. D., Fujimoto, M., Lin, R. P. & Lepping, R. P. In situ detection of collisionless reconnection in the Earth’s magnetotail. Nature 412, 414–417 (2001).
Aschwanden, M. J. Physics of the Solar Corona. An Introduction (Praxis, 2004).
Fletcher, L. et al. An observational overview of solar flares. Space Sci. Rev. 159, 19–106 (2011).
Shibata, K. & Magara, T. Solar flares: Magnetohydrodynamic processes. Living Rev. Solar Phys. 8, 6 (2011).
Chen, P. F. Coronal mass ejections: Models and their observational basis. Living Rev. Solar Phys. 8, 1 (2011).
Yokoyama, T., Akita, K., Morimoto, T., Inoue, K. & Newmark, J. Clear evidence of reconnection inflow of a solar flare. Astrophys. J. 546, L69–L72 (2001).
Li, L. P. & Zhang, J. Observations of the magnetic reconnection signature of an M2 Flare on 2000 March 23. Astrophys. J. 703, 877–882 (2009).
Wang, T. J., Sui, L. H. & Qiu, J. Direct observation of high-speed plasma outflows produced by magnetic reconnection in solar impulsive events. Astrophys. J. 661, L207–L210 (2007).
Savage, S. L. et al. Low-altitude reconnection inflow–outflow observations during a 2010 November 3 solar eruption. Astrophys. J. 754, 13–26 (2012).
Takasao, S., Asai, A., Isobe, H. & Shibata, K. Simultaneous observation of reconnection inflow and outflow associated with the 2010 August 18 solar flare. Astrophys. J. 745, L6 (2012).
Tsuneta, S. et al. Observation of a solar flare at the limb with the YOHKOH Soft X-ray Telescope. Publ. Astron. Soc. Jpn 44, L63–L69 (1992).
Sui, L. H. & Holman, G. D. Evidence for the formation of a large-scale current sheet in a solar flare. Astrophys. J. 596, L251–L254 (2003).
Lin, J. et al. Direct observations of the magnetic reconnection site of an eruption on 2003 November 18. Astrophys. J. 622, 1251–1264 (2005).
Liu, R. et al. A Reconnecting current sheet imaged in a solar flare. Astrophys. J. 723, L28–L33 (2010).
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).
Shibata, K. et al. Observations of X-ray jets with the YOHKOH soft X-ray telescope. Publ. Astron. Soc. Jpn. 44, 173–179 (1992).
Pesnell, W. D., Thompson, B. J. & Chamberlin, P. C. The solar dynamics observatory (SDO). Sol. Phys. 275, 3–15 (2012).
Lemen, J. R. et al. The atmospheric imaging assembly (AIA) on the solar dynamics observatory (SDO). Sol. Phys. 275, 17–40 (2012).
Lin, R. P. et al. The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). Sol. Phys. 210, 3–32 (2002).
Liu, W., Wang, T. J., Dennis, B. R. & Holman, G. D. Episodic X-Ray emission accompanying the activation of an eruptive prominence: Evidence of episodic magnetic reconnection. Astrophys. J. 698, 632–640 (2009).
Longcope, D. W. & Guidoni, S. E. A model for the origin of high density in looptop X-ray sources. Astrophys. J. 740, 73–90 (2011).
Fisher, G. H. & Welsch, B. T. in Subsurface and Atmospheric Influences on Solar Activity, ASP Conference Series Vol. 383 (eds Howe, R., Komm, R. W., Balasubramaniam, K. S. & Petrie, G. J. D.) 373 (Astronomical Society of the Pacific, 2008).
Petschek, H. E. in The Physics of Solar Flares, NASA SP-50 (ed. Hess, W. N.) 425 (NASA, 1964).
Priest, E. R. & Forbes, T. G. The magnetic nature of solar flares. Astron. Astrophys. Rev. 10, 313–377 (2002).
Schmahl, E. J., Pernak, R. L., Hurford, G. J., Lee, J. & Bong, S. Analysis of RHESSI flares using a radio astronomical technique. Sol. Phys. 240, 241–252 (2007).
Neupert, W. M. comparison of solar x-ray line emission with microwave emission during flares. Astrophys. J. 153, L59 (1968).
Acknowledgements
The authors dedicate this paper to the late RHESSI PI, R. P. Lin, in acknowledgement of his inspirational efforts that made possible the high-quality solar X-ray data used in this paper. RHESSI is a NASA Small Explorer Mission. The GOES is a joint effort of NASA and the National Oceanic and Atmospheric Administration (NOAA). The SDO is a mission for NASA’s Living With a Star (LWS) Program. The work of Y.S. and A.M.V. was supported by the European Community Framework Programme 7, High Energy Solar Physics data in Europe (HESPE), grant agreement No. 263086. Y.S. also acknowledges NSFC 11233008. The work of G.H. was supported by a NASA Guest Investigator Grant and the RHESSI programme. The work of T.W. was supported by NASA grant NNX12AB34G and NASA Cooperative Agreement NNG11PL10A to C.U.A. M.T. acknowledges the Austrian Science Fund (FWF): V195-N16. W.G. acknowledges 2011CB811402 and NSFC 11233008.
Author information
Authors and Affiliations
Contributions
Y.S. analysed the data, wrote the text and led the discussion. A.M.V., G.D.H., B.R.D., T.W., M.T. and W.G. contributed to the interpretation of the data and helped to improve the manuscript.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing financial interests.
Supplementary information
Supplementary Information
Supplementary Information (PDF 511 kb)
Supplementary Movie
Supplementary movie 1 (MOV 17427 kb)
Supplementary Movie
Supplementary movie 2 (MOV 19319 kb)
Supplementary Movie
Supplementary movie 3 (MOV 18683 kb)
Supplementary Movie
Supplementary movie 4 (MOV 9151 kb)
Supplementary Movie
Supplementary movie 5 (MOV 6359 kb)
Rights and permissions
About this article
Cite this article
Su, Y., Veronig, A., Holman, G. et al. Imaging coronal magnetic-field reconnection in a solar flare. Nature Phys 9, 489–493 (2013). https://doi.org/10.1038/nphys2675
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/nphys2675
This article is cited by
-
Circular-ribbon flares and the related activities
Reviews of Modern Plasma Physics (2024)
-
Ultra-high-resolution observations of persistent null-point reconnection in the solar corona
Nature Communications (2023)
-
The origin of underdense plasma downflows associated with magnetic reconnection in solar flares
Nature Astronomy (2022)
-
Fast plasmoid-mediated reconnection in a solar flare
Nature Communications (2022)
-
Invited Review: Short-term Variability with the Observations from the Helioseismic and Magnetic Imager (HMI) Onboard the Solar Dynamics Observatory (SDO): Insights into Flare Magnetism
Solar Physics (2022)