D. Jess et al.

The elusive Alfvén wave has been at the centre of atmospheric heating studies for decades, due to its unique incompressible nature making it an ideal energy transporter. However, the viability of Alfvén waves as a heating mechanism in the chromosphere, and beyond, depends on efficient thermalisation of the wave energy, which has previously been undetectable. We combine high resolution spectral imaging with advanced inversion techniques to uncover the first evidence of Alfvén wave energy conversion, in the form of non-linear shock fronts in a sunspot umbra. Observed local temperature enhancements of 5% are observed in regions where conventional umbral flash formation is inhibited, providing a unique and unprecedented insight into Alfvén wave behaviour in the solar atmosphere. Importantly, this work also provides avenues for future research with next generation telescopes and observations.