Alfv\'en Wave Dissipation in the Solar Chromosphere

TL;DR

This paper presents the first observational evidence that Alfvén waves can heat the solar chromosphere by dissipating energy through shock formation, advancing understanding of solar atmospheric heating mechanisms.

Contribution

It provides direct observational evidence of Alfvén wave dissipation heating the chromosphere, a key step in confirming their role in solar atmospheric energy transfer.

Findings

Detected local temperature increases of 5% in sunspot umbrae.

Identified shock fronts associated with Alfvén wave dissipation.

Distinguished shock signatures from conventional umbral flashes.

Abstract

Magneto-hydrodynamic (MHD) Alfv\'en waves have been a focus of laboratory plasma physics and astrophysics for over half a century. Their unique nature makes them ideal energy transporters, and while the solar atmosphere provides preferential conditions for their existence, direct detection has proved difficult as a result of their evolving and dynamic observational signatures. The viability of Alfv\'en waves as a heating mechanism relies upon the efficient dissipation and thermalization of the wave energy, with direct evidence remaining elusive until now. Here we provide the first observational evidence of Alfv\'en waves heating chromospheric plasma in a sunspot umbra through the formation of shock fronts. The magnetic field configuration of the shock environment, alongside the tangential velocity signatures, distinguish them from conventional umbral flashes. Observed local temperature enhancements of 5% are consistent with the dissipation of mode-converted Alfv\'en waves driven by upwardly propagating magneto-acoustic oscillations, providing an unprecedented insight into the behaviour of Alfv\'en waves in the solar atmosphere and beyond.