Acoustic oscillations in a field-free cavity under solar small-scale bipolar magnetic canopy

TL;DR

This paper investigates how acoustic waves interact with small-scale bipolar magnetic canopies in the solar atmosphere, explaining observed high-frequency power enhancements through wave trapping mechanisms.

Contribution

It introduces an analytical model of acoustic oscillations in field-free cavities beneath magnetic canopies, revealing how certain wave modes are trapped and contribute to observed phenomena.

Findings

Higher azimuthal modes ($m \\geq 2$) are trapped under the canopy.

The $m=1$ mode cannot be trapped due to energy leakage.

The model explains the formation of acoustic power halos.

Abstract

Observations show the increase of high-frequency wave power near magnetic network cores and active regions in the solar lower atmosphere. This phenomenon can be explained by the interaction of acoustic waves with a magnetic field. We consider small-scale, bipolar, magnetic field canopy structure near the network cores and active regions overlying field-free cylindrical cavities of the photosphere. Solving the plasma equations we get the analytical dispersion relation of acoustic oscillations in the field-free cavity area. We found that the m = 1 mode, where m is azimuthal wave number, cannot be trapped under the canopy due to energy leakage upwards. However, higher ($m \geq 2$) harmonics can be easily trapped leading to the observed acoustic power halos under the canopy.