The Sun at millimeter wavelengths IV. Magnetohydrodynamic waves in small-scale bright features

TL;DR

This study combines ALMA observations and numerical simulations to analyze small-scale magnetic features in the solar chromosphere, revealing oscillatory MHD waves and estimating their energy flux, suggesting wave heating processes.

Contribution

It provides the first detailed analysis of magnetohydrodynamic waves in small-scale bright features using combined observational and simulation data.

Findings

Detected oscillatory behavior in temperature, size, and velocity of features.

Identified possible transverse (kink) waves with amplitudes of 2.1-5.0 km/s.

Estimated wave energy flux indicating potential wave heating in the chromosphere.

Abstract

We used solar observations of a plage/enhanced network with the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Band 3 and Band 6 together with synthetic continuum maps from numerical simulations with Bifrost at the same bands to carry out a detailed study of bright small-scale magnetic features. To this end, we have used an algorithm to automatically identify and trace the features within the field of view (FoV) of the observations and the simulation. We found 193 and 293 features in the Bands 3 and 6 observations, respectively. In the degraded simulation, the total number of features were 24 for Band 3 and 204 for Band 6. In the original simulation, the total number of features were 36 for Band 3 and 392 for Band 6. Based on the simulation, we confirm the magnetic nature of the features which exhibit an oscillatory behaviour in temperature, size and horizontal velocity. The average oscillation periods were of 30-99\,s for temperature, 37-92\,s for size and 37-78\,s for horizontal velocity. There are indications for the possible presence of transverse (kink) waves with average amplitude velocities of 2.1-5.0\,km\,s$^{-1}$. An anti-phase behaviour between temperature and size oscillations suggest the presence of compressible fast-sausage Magnetohydrodynamics (MHD) modes. Finally, we have estimated the flux of energy of the fast-sausage waves at the chromospheric heights sampled by ALMA as 453-1838\,W\,m$^{-2}$ for Band 3 and 3640-5485\,W\,m$^{-2}$ for Band 6. The decrease of wave energy-flux with height (from Band 6 to Band 3) could possibly suggest energy dissipation at chromospheric heights, thus wave heating, with the assumptions that the identified small-scale waves are typical at each band and they propagate upward through the chromosphere.