The Frequency-dependent Damping of Slow Magnetoacoustic Waves in a Sunspot Umbral Atmosphere

TL;DR

This study investigates how slow magnetoacoustic waves in a sunspot umbra are damped in a frequency-dependent manner, revealing propagation characteristics, energy flux decay, and spatial variations in wave power.

Contribution

It provides new insights into the frequency-dependent damping and propagation of slow magnetoacoustic waves in sunspot atmospheres using high-resolution multi-wavelength observations.

Findings

Power spectra follow a power-law dependence across frequencies.

Higher frequency oscillations have shorter damping lengths.

Energy flux of waves decreases with height, consistent with simulations.

Abstract

High spatial and temporal resolution images of a sunspot, obtained simultaneously in multiple optical and UV wavelengths, are employed to study the propagation and damping characteristics of slow magnetoacoustic waves up to transition region heights. Power spectra are generated from intensity oscillations in sunspot umbra, across multiple atmospheric heights, for frequencies up to a few hundred mHz. It is observed that the power spectra display a power-law dependence over the entire frequency range, with a significant enhancement around 5.5 mHz found for the chromospheric channels. The phase-difference spectra reveal a cutoff frequency near 3 mHz, up to which the oscillations are evanescent, while those with higher frequencies propagate upwards. The power-law index appears to increase with atmospheric height. Also, shorter damping lengths are observed for oscillations with higher frequencies suggesting frequency-dependent damping. Using the relative amplitudes of the 5.5 mHz (3 minute) oscillations, we estimate the energy flux at different heights, which seems to decay gradually from the photosphere, in agreement with recent numerical simulations. Furthermore, a comparison of power spectra across the umbral radius highlights an enhancement of high-frequency waves near the umbral center, which does not seem to be related to magnetic field inclination angle effects.