The temperature of the Sun's outer atmosphere — the corona — can reach a few million degrees, making it even hotter than the Sun's surface, or photosphere. Two different mechanisms could be behind the coronal heating: the plasma and magnetic-field oscillations know as Alfvén waves; and magnetic 'braids', which are reconnected magnetic-field loops.

Alfvén wave-heating, the main heating mechanism in quiet periods of solar activity, contributes to a rise in coronal temperature of about 1.5 million kelvin. The extra energy required to reach temperatures of up to 4 million kelvin in the active regions may arise from the dynamics of magnetic braids, but so far direct evidence of braiding in the corona has been lacking.

As they report in Nature, Jonathan Cirtain and colleagues used new data from NASA's High Resolution Coronal Imager (Hi-C, a rocket-borne telescope launched in July 2012) to resolve magnetic braids whose dissipated energy could account for the high coronal temperatures (Nature 493, 501–503; 2013).

AIA/SDO/NASA

Observations from JAXA's Hinode spacecraft and from the Atmospheric Imaging Assembly (AIA) onboard NASA's Solar Dynamics Observatory have provided information about coronal temperatures, but the data lacks the resolution to distinguish the twisting and braiding of magnetic-field loops. Hi-C, however, has captured the most detailed images of the corona ever taken — 150-kilometre resolution at extreme ultraviolet wavelengths, producing pictures five times more detailed than those from the AIA.

Cirtain et al. are able to clearly identify the twisting, reconnecting dance of magnetic-field loops. The lively dynamics — driven by convective flows in the photosphere — release energy into the corona, in sufficient quantities to raise the temperature to 4 million kelvin. However, more extensive future observations will determine whether magnetic braiding is the dominant heating mechanism in the active regions.