Benchmarking Fast-to-Alfv\'en Mode Conversion in a Cold MHD Plasma. II. How to get Alfv\'en waves through the Solar Transition Region

TL;DR

This paper investigates how Alfvén waves can be generated in the solar chromosphere through mode conversion from fast MHD waves, reducing reflection at the transition region and impacting seismic diagnostics.

Contribution

It demonstrates that high-altitude mode conversion facilitates Alfvén wave transmission through the transition region, offering a new mechanism for their presence in the corona.

Findings

Alfvén waves can be generated high in the chromosphere via mode conversion.

Fast wave reflection is minimized when the reflection point is near the transition region.

Phase changes in reflected fast waves can affect seismic measurements.

Abstract

Alfv\'en waves may be difficult to excite at the photosphere due to low ionization fraction and suffer near-total reflection at the transition region (TR). Yet they are ubiquitous in the corona and heliosphere. To overcome these difficulties, we show that they may instead be generated high in the chromosphere by conversion from reflecting fast magnetohydrodynamic waves, and that Alfv\'enic transition region reflection is greatly reduced if the fast reflection point is within a few scale heights of the TR. The influence of mode conversion on the phase of the reflected fast wave is also explored. This phase can potentially be misinterpreted as a travel speed perturbation, with implications for the practical seismic probing of active regions.