The Sun at millimeter wavelengths V. Magnetohydrodynamic waves in a fibrillar structure

TL;DR

This study uses high-resolution ALMA observations to detect and analyze MHD waves in a solar fibril, revealing both standing and propagating wave modes and demonstrating ALMA's effectiveness in studying dynamic chromospheric structures.

Contribution

First detection of MHD wave modes in a fibril using ALMA, providing detailed wave characteristics and confirming ALMA's capability for chromospheric wave studies.

Findings

Detected oscillations with median periods around 240 seconds.

Identified both standing and propagating MHD kink and sausage modes.

Measured phase speeds of propagating waves up to 74 km/s.

Abstract

Magnetohydrodynamic (MHD) waves, playing a crucial role in transporting energy through the solar atmosphere, manifest in various chromospheric structures. Here, we investigated MHD waves in a long-lasting dark fibril using high-temporal-resolution (2~s cadence) Atacama Large Millimeter/submillimeter Array (ALMA) observations in Band 6 (centered at 1.25~mm). We detected oscillations in brightness temperature, horizontal displacement, and width at multiple locations along the fibril, with median periods and standard deviations of $240\pm114$~s, $225\pm102$~s, and $272\pm118$~s, respectively. Wavelet analysis revealed a combination of standing and propagating waves, suggesting the presence of both MHD kink and sausage modes. Less dominant than standing waves, oppositely propagating waves exhibit phase speeds (median and standard deviation of distributions) of $74\pm204$~km/s, $52\pm197$~km/s, and $28\pm254$~km/s for the three observables, respectively. This work demonstrates ALMA's capability to effectively sample dynamic fibrillar structures, despite previous doubts, and provides valuable insights into wave dynamics in the upper chromosphere.