Energy release in the solar corona from spatially resolved magnetic braids

It is now apparent that there are at least two heating mechanisms in the Sun's outer atmosphere, or corona(1-5). Wave heating may be the prevalent mechanism in quiet solar periods and may contribute to heating the corona to 1,500,000K (refs 1-3). The active corona needs additional heating to reach 2,000,000-4,000,000 K; this heat has been theoretically proposed(6-12) to come from the reconnection and unravelling of magnetic 'braids'. Evidence favouring that process has been inferred(13,14), but has not been generally accepted because observations are sparse and, in general, the braided magnetic strands that are thought(1-3,15-17) to have an angular width of about 0.2 arc seconds have not been resolved(10,18-20). Fine-scale braiding has been seen(21,22) 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 4,000,000K. 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.