The atmosphere of the Sun is an incredibly complex environment shaped by the ever-evolving solar magnetic field. But this complexity has its limits; when the magnetic fields tangle together and their energy increases, magnetic reconnection sets on so a lower-energetic state can be reached. While reconnection itself cannot be observed directly, its signatures are ubiquitous. Prime examples of reconnection’s doing are solar flares, powerful explosions heating solar plasmas to tens of millions of degrees and releasing as much as 10²⁵ joules of energy, equivalent to billions of megatons of TNT, into the interplanetary space.

Numerical simulations revealed that magnetic reconnection enables magnetic field line to quickly slide past each other, so-called slipping motions [1]. They are observed in magnetic loops (see Animation 1) as well as kernels, small bright patches at anchors of the loops [2] best seen in the ultraviolet (UV) part of the spectrum. However, no previous analyses managed to observe these motions at speeds predicted by the simulations [3] — reaching thousands of kilometers per second and more. This changed when NASA’s Interface Region Imaging Spectrograph (IRIS) started to capture flare images at a very high temporal resolution, in some cases at sub-second rates.

Our new analysis focuses on UV observations of an otherwise very ordinary flare from September 2022 (Figure, top). Mere animations of these observations (Animation 2) indicate the presence of rapidly moving kernels, appearing as patches of emission (in black) along a flare ribbon: elongated "footprint" of the flare. The dynamics of the kernels were measured using time-distance diagrams (Figure, bottom), used to track the evolution of emission in time along a pre-defined slice (pink arrow in Figure, top). Slipping kernels in this diagram imprint slanted traces whose inclination is proportional to the kernel velocities: the higher the inclination of a trace, the faster the kernel. Example kernel traces have been highlighted using the red rectangles in the time-distance diagram. Linear fits to kernel traces as well as computer vision technique for feature detection evidenced that the kernels were indeed extremely fast, in some cases slipping at speeds well over 1000 km/s. This observation is the missing piece of evidence of slip-running regime of magnetic reconnection, elusive in observations for nearly two decades. Interestingly, the term “slip-running reconnection” was coined by our collaborator Guillaume Aulanier back in 2006. He was inspired by the comic character Wile E. Coyote, who, equipped with a fork and a knife, chased after the Road Runner so fast that his legs were “slip-running”.

Employing computer vision algorithms has another advantage. We find that, had the kernels been observed at the temporal resolution of just over 4 seconds, the traces of the fastest kernels would completely disappear from data! This proves that the temporal resolution of IRIS was indispensable in this discovery, setting important constraints on the performance of future solar instrumentation aimed to observe dynamic phenomena such as flares. Speaking of the future, this discovery needs to be expanded upon, particularly by analyzing a multitude of solar flares of different strengths and magnetic environments. Magnetic reconnection is a fundamental plasma physics process that takes place in a variety of astrophysical settings; including of course stellar flares and eruptions, planetary magnetic fields producing spectacular aurorae, as well as accretion disks of black holes. 

_{[1] - Aulanier, G. et al. “Slip-running reconnection in quasi-separatrix layers”, Solar Physics, 238, 347 (2006)
[2] - Dudík, J. et al. “Slipping magnetic reconnection during an X-class solar flare observed by SDO/AIA”, The Astrophysical Journal, 784, 144 (2014)
[3] - Janvier, M. et al. “The standard flare model in three dimensions: III. Slip-running reconnection properties”, Astronomy & Astrophysics, 555, A77 (2013)}