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Energy release in the solar corona from spatially resolved magnetic braids

Abstract

It is now apparent that there are at least two heating mechanisms in the Sun’s outer atmosphere, or corona1,2,3,4,5. Wave heating may be the prevalent mechanism in quiet solar periods and may contribute to heating the corona to 1,500,000 K (refs 1, 2, 3). The active corona needs additional heating to reach 2,000,000–4,000,000 K; this heat has been theoretically proposed6,7,8,9,10,11,12 to come from the reconnection and unravelling of magnetic ‘braids’. Evidence favouring that process has been inferred13,14, but has not been generally accepted because observations are sparse and, in general, the braided magnetic strands that are thought1,2,3,15,16,17 to have an angular width of about 0.2 arc seconds have not been resolved10,18,19,20. Fine-scale braiding has been seen21,22 in the chromosphere but not, until now, in the corona. Here we report observations, at a resolution of 0.2 arc seconds, of magnetic braids in a coronal active region that are reconnecting, relaxing and dissipating sufficient energy to heat the structures to about 4,000,000 K. Although our 5-minute observations cannot unambiguously identify the field reconnection and subsequent relaxation as the dominant heating mechanism throughout active regions, the energy available from the observed field relaxation in our example is ample for the observed heating.

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Figure 1: The 1.5-MK sun.
Figure 2: A coronal loop seen at several different coronal temperatures by AIA and Hi-C.
Figure 3: A time series from Hi-C data.
Figure 4: The light curves for example two.

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Acknowledgements

We thank the NASA Low-Cost Access to Space programme for supporting the development of the Hi-C instrument, the NASA Sounding Rocket Office for the launch of the instrument and the NASA Marshall Space Flight Center for instrument development support. This LPI work was supported in part by the Russian Foundation for Basic Research (project 11–02–01079-a), Program No. 22 of the Presidium of the Russian Academy of Sciences.

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Contributions

J.W.C., A.R.W., B.D.P., P.M. and C.E.D. performed image processing and analysis of observations. M.W., K.K., B.D.P., C.E.D. and A.R.W. all contributed to the calibration and alignment of the instrument and science data. J.W.C., L.G., K.K. and K.E.K. managed the design, construction and testing of the experiment. J.W.C. and A.R.W. determined velocities and J.W.C and R.L.M. calculated the stored energy. A.T., R.W.W. and S.K. contributed to the instrumentation.

Corresponding author

Correspondence to J. W. Cirtain.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Info

DESCRIPTION (PDF 2220 kb)

Experiment regions of interest

This video shows the full sun AIA images taken during the Hi-C flight. It zooms into the Hi-C field of full field of view and extracts the two examples from the paper, and shows comparison movies for AIA and Hi-C. The first comparison is for example 1 shown in Fig 2 and Supplementary Fig. 2 and Supplementary Video 2. The second comparison is for example 2 and goes with Fig. 3 and Supplementary Videos 3 & 4. (MOV 4672 kb)

Evolution of loops during component reconnection

This video presents the evolution of a set of loops that are interacting and wrapped together. As the structures evolve, reconnection allows the loops to relax into a less curved and thus more potential geometry. (MOV 4160 kb)

Evolution in a Braided Loop Ensemble

This braided loop has several loops near the 'base' that appear to be unwinding with significant apparent outflow. This is evidence of untwisting, and the braided structure also seeming to unwind with time. (MOV 5888 kb)

The Temperature history of the Braided Structure

Within the braided bundle, several episodic intensity increases are observed in many of the AIA passbands. This is direct evidence of impulsive energy release likely the result of reconnection. The process is observed many times in the AIA data over a long period of observations leading to the conclusion that the region is constantly being sheared, thus storing energy, and frequently releases some of this energy in these localized heating events. (MOV 1664 kb)

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Cirtain, J., Golub, L., Winebarger, A. et al. Energy release in the solar corona from spatially resolved magnetic braids. Nature 493, 501–503 (2013). https://doi.org/10.1038/nature11772

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