First direct observation of a torsional Alfv\'en oscillation at coronal heights

Torsional Alfv'en waves are promising candidates for transport of energy across different layers of the solar atmosphere and have been theoretically predicted for decades. Previous detections of Alfv'en waves so far have however mostly relied on indirect signatures. We present a first direct observational evidence of a fully resolved torsional Alfv'en oscillation of a large-scale structure occurring at coronal heights. We analyse IRIS imaging and spectral observation of a surge resulting from magnetic reconnection between active region prominence threads and surrounding magnetic fieldlines. The IRIS spectral data provides clear evidence of an oscillation in the line-of-sight velocity with a 180{\deg} 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 provides clear evidence of torsional oscillation of the flux tube. Our observation shows that magnetic reconnection leads to the generation of large-scale torsional Alfv'en waves.