Vol. 633
Sect. 1. Letters

First direct observation of a torsional Alfvén oscillation at coronal heights

The magnetized solar atmosphere can support a variety of waves. Torsional, or Alfvén waves, are promising candidates for transporting energy across different layers of the solar atmosphere and have been theoretically predicted for decades. Previous detections of Alfvén waves have, however, so far relied mostly on indirect signatures. Using data from the space-based IRIS observatory, the authors analyzed imaging and spectral observation of a surge of cool solar plasma which is triggered by a magnetic reconnection between open and closed magnetic field lines happening in the solar corona. The IRIS spectral data shows an oscillation in the line-of-sight velocity with a 180° phase difference at opposite edges of the surge flux tube. This signature of torsional oscillation is further complimented with an alternating tilt observable 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. Together these provide the first direct observational evidence that magnetic reconnection leads to the generation of large-scale torsional Alfvén waves in the solar corona.