Slipping motions of magnetic field lines are a distinct signature of three-dimensional magnetic reconnection, a fundamental process driving solar and stellar flares. While being a key prediction of numerical experiments, the rapid super-Alfvénic field line slippage driven by the 'slip-running' reconnection has remained elusive in previous observations. New frontiers into exploring transient flare phenomena were introduced by recently designed high cadence observing programs of the Interface Region Imaging Spectrograph (IRIS). By exploiting high temporal resolution imagery (~2 s) of IRIS, here we reveal slipping motions of flare kernels at speeds reaching thousands of kilometres per second. The fast kernel motions are direct evidence of slip-running reconnection in quasi-separatrix layers, regions where magnetic field strongly changes its connectivity. Our results provide observational proof of theoretical predictions unaddressed for nearly two decades and extend the range of magnetic field configurations where reconnection-related phenomena can occur.

Astronomical Institute of the Czech Academy of Sciences Ondřejov Czech Republic

Bay Area Environmental Research Institute NASA Research Park Moffett Field CA USA

Department of Physics Oregon State University Corvallis OR USA

Institute of Theoretical Astrophysics University of Oslo Blindern Norway

Lockheed Martin Solar and Astrophysics Laboratory Palo Alto CA USA

Rosseland Center for Solar Physics University of Oslo Blindern Norway

Sorbonne Université Observatoire de Paris PSL École Polytechnique Institut Polytechnique de Paris CNRS Laboratoire de physique des plasmas Paris France

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