Evidence for small-scale torsional Alfvén waves in the solar corona

The corona is the outermost layer of the Sun's atmosphere. Its plasma is accelerated and flows out into interplanetary space as a heated, supersonic wind. The details of energy and momentum transfer to the plasma remain debated. Alfvén waves are a favoured mechanism, and in a plasma composed of inhomogeneous flux tubes, the only pure Alfvén mode is torsional in nature. Large-scale modes have been observed sporadically, but a prevalent, small-scale counterpart in the corona has yet to be established. The Daniel K. Inouye Solar Telescope has begun to provide unprecedented views of the Sun, with the Cryo-NIRSP instrument delivering coronal observations with a high spatial and spectral resolution. Here the data reveal that the quiescent corona supports torsional Alfvén waves, which continuously twist the magnetic field lines back and forth. The measured wave amplitudes are small but are probably underestimated due to the line-of-sight integration. The results indicate that the waves may carry a substantial fraction of the energy required to power the quiet Sun and solar wind.