Solar coronal heating from small-scale magnetic braids

Vol. 667
9. The Sun and the Heliosphere

Solar coronal heating from small-scale magnetic braids

The Sun’s outer atmosphere is dominated by million-kelvin hot plasma, magnetically confined in arches with footpoints in the 6000 K cool photosphere. These coronal loops are abundant in active regions, that is, in areas that often host sunspots. How the plasma in coronal loops is heated to such high temperatures is a major topic of debate in stellar astrophysics. Photospheric convective motions continuously tangle and braid coronal magnetic fields. In one of the widely invoked heating models of loops, such coronal magnetic braids are thought to untangle and relax through reconnection and to transfer energy to heat the plasma. Earlier studies based on extreme ultraviolet observations of the solar corona hinted that coronal magnetic braids might operate on small spatial scales of less than 1000 km. However, direct imaging observations of relaxing braids and subsequent coronal heating remain sparse. Using new high spatial resolution (250-270 km on the Sun) and high cadence (3-10 s) observations from the Extreme Ultraviolet Imager (EUI) on board ESA/NASA Solar Orbiter, the authors observed the untangling of small-scale coronal braids and plasma heating in different active regions. Their observations indicate that magnetic reconnection relaxing coronal braids proceeds on timescales of a few tens of seconds to a few hundred seconds, giving rise to signatures of both spatially coherent and intermittent coronal heating. Thanks to the increased spatial resolution and higher cadence of the EUI observations, now more of these events can be investigated to test the hypothesis that this braiding is or is not a major contributor to coronal heating.