On Mode Conversion, Reflection and Transmission of Magneto-Acoustic Waves from Above in an Isothermal Stratified Atmosphere

TL;DR

This paper analyzes how magnetoacoustic waves in an isothermal stratified atmosphere can penetrate magnetic canopies, potentially explaining the energy transfer responsible for sunquakes, through exact solutions and wave conversion mechanisms.

Contribution

It provides an exact analytical framework for understanding wave reflection, transmission, and conversion in a stratified magnetic atmosphere, relevant to solar flare energy transport.

Findings

Slow waves efficiently transmit as acoustic waves at small attack angles.

Low-frequency waves transmit due to the ramp effect in inclined fields.

Evanescent fast waves can tunnel and convert to acoustic waves reaching the photosphere.

Abstract

We use the exact solutions for magnetoacoustic waves in a two dimensional isothermal atmosphere with uniform inclined magnetic field to calculate the wave reflection, transmission, and conversion of slow and fast waves incident from above ($z=\infty$). This is relevant to the question of whether waves excited by flares in the solar atmosphere can penetrate the Alfv\'en/acoustic equipartition layer (which we identify as the canopy) to reach the photosphere with sufficient energy to create sunquakes. It is found that slow waves above the acoustic cutoff frequency efficiently penetrate (transmit) as acoustic (fast) waves if directed at a small attack angle to the magnetic field, with the rest converting to magnetic (slow) waves, in accord with Generalized Ray Theory. This may help explain the compact nature of seismic sources of sunquakes identified using seismic holography. The incident slow waves can also efficiently transmit at low frequency in inclined field due to the reduction in acoustic cutoff frequency (ramp effect). Incident fast (magnetic) "waves" from infinity with specified nonzero horizontal wavenumber are necessarily evanescent, but can carry energy to the equipartition level by tunnelling. It is found that this can then efficiently convert to acoustic (fast) energy that can again reach the photosphere as a travelling wave. Overall, there appear to be ample avenues for substantial compressive wave energy to penetrate the canopy and impact the photosphere.