First direct observation of a torsional Alfvén oscillation at coronal heights

Context. Torsional Alfven waves are promising candidates for the transport of energy across different layers of the solar atmosphere. They have been predicted theoretically for decades. Previous detections of Alfven waves so far have however mostly relied on indirect signatures. Aims. We present the first direct observational evidence of a fully resolved torsional Alfven oscillation of a large-scale structure occurring at coronal heights. Methods. We analysed IRIS imaging and spectral observation of a surge resulting from magnetic reconnection between active region prominence threads and surrounding magnetic field lines. Results. The IRIS spectral data provide clear evidence of an oscillation in the line-of-sight velocity with a 180 degrees phase difference between the oscillation signatures at opposite edges of the surge flux tube. This together with an alternating tilt in the Si IV and Mg II k spectra across the flux tube and the trajectories traced by the individual threads of the surge material provide clear evidence of torsional oscillation of the flux tube. Conclusions. Our observation shows that magnetic reconnection leads to the generation of large-scale torsional Alfven waves.