The Excitation of Coronal Alfvénic Waves by the Internal Acoustic Modes

Abstract

Sunspots are intense regions of magnetic flux that are rooted deep below the photosphere. It is well established that sunspots host magnetohydrodynamic waves, with numerous observations showing a connection to the internal acoustic (or p-)modes of the Sun. The p-modes are fast waves below the equipartition layer and are thought to undergo a double mode conversion as they propagate upward into the atmosphere of sunspots, which can generate transverse, magnetic-tension-driven, and highly incompressible wave modes called the Alfvénic waves in the upper atmosphere. We employ 2.5D magnetohydrodynamic (MHD) numerical simulations to investigate the adiabatic wave propagation and to examine the resulting power spectra of coronal Alfvénic waves. A broadband wave source is used that has a 1D power spectrum, which mimics aspects of the observed p-mode power spectrum. We examine the propagation of magnetoacoustic waves and the conversion of modes from the photosphere to the corona. Frequency filtering of the upwardly propagating acoustic waves is a natural consequence of a gravitationally stratified atmosphere and plays a key role in shaping the power spectra of mode converted waves. We demonstrate that the slow, the fast magnetoacoustic waves and Alfvénic waves above the equipartition layer have similarly shaped power spectra, which are modified versions of the driver spectrum. Notably, the results reveal that the coronal wave power spectra have a peak at a higher frequency than that of the underlying p-mode driver. This matches observations of coronal Alfvénic waves and further supports the role of mode conversion process as a mechanism for Alfvénic wave generation in the Sun’s atmosphere. An in-depth exploration of the modified power in coronal MHD waves when driven by a p-mode driver is needed, as MHD mode conversion is a linear process that does not affect the frequency of converted waves. In order to prove the underlying phenomenon behind the modified power spectra of Alfvénic waves is caused by the background cut-off frequency that naturally arises due to gravitational stratification, we make use of three monochromatic simulations, each driven by 3, 5, 10 mHz vertical drivers, respectively. This experiment proves the absence of chromospheric resonant cavity effects on coronal Alfvénic waves within our simulations. The effects of ambipolar diffusion on double mode conversion are also evaluated using a 3 mHz monochromatic wave source on a much larger sunspot domain. The preliminary results show that ambipolar diffusion not only does not influence double mode conversion but also does not alter the coronal power spectra of the coronal MHD waves arising from the acoustic cut-off.

Date of Award	22 May 2025
Original language	English
Awarding Institution	Northumbria University
Supervisor	Richard Morton (Supervisor), Patrick Antolin (Supervisor) & James McLaughlin (Supervisor)