On the origins of coronal Alfv\'enic waves

TL;DR

This study uses high-resolution DKIST observations to analyze the coronal Alfvénic wave power spectrum, confirming their excitation by photospheric motions and highlighting the roles of reflection, dissipation, and additional sources in coronal heating.

Contribution

It provides the first detailed coronal Alfvénic wave power spectrum from DKIST data, supporting models of wave excitation and dissipation in the solar atmosphere.

Findings

Coronal Alfvénic power spectrum shows a broad enhancement.

Low-frequency waves are reflected at the transition region.

Higher frequency waves are dissipated by the chromosphere.

Abstract

Alfv\'enic waves are considered a key contributor to the energy flux that powers the Sun's corona, with theoretical models demonstrating their potential to explain coronal EUV and X-ray emission and the acceleration of the solar wind. However, confirming underlying assumptions of the models has proved challenging, especially obtaining evidence for the excitation and dissipation of Alfv\'enic waves in the lower solar atmosphere and tracing their propagation into the corona. We present an investigation of the Alfv\'enic wave power spectrum in the Sun's corona, obtained from observations with DKIST Cryo-NIRSP. The data provide unprecedented temporal resolution and signal-to-noise, revealing a detailed power spectrum out to frequencies exceeding 10 mHz. A broad enhancement in power dominates the spectrum and we demonstrate it is accurately reproduced using a physics-based model. The results corroborate the scenario where the corona is dominated by Alfv\'enic waves excited in the photosphere by horizontal convective motions, with low-frequency waves subject to reflection at the transition region and higher frequency waves significantly dissipated by the partially ionized chromosphere. The coronal Alfv\'enic power spectrum also indicates there are contributions from \textit{p}-modes (via mode conversion) and a yet-unknown higher-frequency source. These results provide key insight into how the Sun's convective motions imprint themselves on the corona and highlight the critical role of partial ionization, reflection, and damping in regulating upward-propagating Alfv\'enic waves. A further implication of this is that reconnection-driven Alfv\'enic waves likely play a smaller role in powering the corona and solar wind than has been suggested by recent studies.