MHD wave transmission in the Sun's atmosphere

TL;DR

This study investigates MHD wave transmission in the Sun's atmosphere, revealing how magnetic topology influences wave propagation, transmission peaks related to magnetic inclination, and evidence of longitudinal acoustic waves along magnetic field lines.

Contribution

It provides new observational insights into wave transmission at different atmospheric heights and proposes a simple model explaining transmission peaks based on magnetic inclination.

Findings

Substantial leakage of 5-minute waves at pore edges.

Transmission peaks at 25° for 5-minute and 15° for 3-minute waves.

Evidence of longitudinal acoustic waves along magnetic field lines.

Abstract

MHD wave propagation inside the Sun's atmosphere is closely related to the magnetic field topology. For example, magnetic fields are able to lower the cutoff frequency for acoustic waves thus allowing waves which would otherwise be trapped below the photosphere to propagate into the upper atmosphere. Another example is that MHD waves can be transmitted or converted into other forms of waves at altitudes where the sound speed equals the Alfv\'en speed. We take advantage of the large field-of-view provided by the IBIS experiment to study the wave propagation at two heights in the solar atmosphere, as sampled by the photospheric Fe 617.3 nm spectral line and the chromospheric Ca 854.2 nm spectral line, and its relationship to the local magnetic field. Among other things, we find substantial leakage of waves with 5-minute periods in the chromosphere at the edges of a pore and in the diffused magnetic field surrounding it. By using spectro-polarimetric inversions of Hinode SOT/SP data, we also find a relationship between the photospheric power spectrum and the magnetic field inclination angle; in particular, well-defined transmission peaks around 25\degree for 5 minutes waves and around 15\degree for 3 minutes waves. We propose a very simple model based upon wave transmission theory to explain this behavior. Finally, an analysis of both the power spectra and chromospheric amplification spectra suggests the presence of longitudinal acoustic waves along the magnetic field lines.