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

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 upward. 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.

A chromospheric resonance cavity in a sunspot mapped with seismology

“…3a point towards a universal sunspot characteristic for waves detected in chromospheric spectral lines. A recent observational examination of the spectral power slopes for an entirely different sunspot, using ground-based images obtained in the Ca II K and Hα line cores, found similar spectral gradients (within the 6 − 17 mHz spectral range) to those presented here 42 . These gradients are steeper than both the f −1 and f −5/3 relationships that would be expected for granulation (i.e., pink) noise patterns 46 and the Kolomogorov inertial range 47 , respectively.…”

“…To calculate the spectral slopes corresponding to regions II and III, maximum-likelihood fitted gradients are computed for each of the 101 spectral energies across the chromospheric umbra. Often linear lines of best fit are established to determine the spectral slopes of Fourier power spectra 42 . However, the weighted least-squares minimisation process assumes that the data to be fitted are normally (Gaussian) distributed 20 , which may not necessarily be the case, especially when the periodogram of a stationary, linear stochastic process naturally follows a χ 2 2 distribution 43,44 .…”

Wave amplitude modulation in fan loops as observed by AIA/SDO

“…They related such highfrequency modes at coronal heights to perturbations at photospheric umbra. Krishna Prasad et al (2017) reported frequencies around 13 mHz using IRIS 2796 Å in chromospheric sunspot umbra. We note that the observed high frequency around 11 mHz is close to two times that of the dominant (fundamental) frequency at around 5.56 mHz, and hence is similar to the second harmonics.…”

Spectropolarimetric fluctuations in a sunspot chromosphere

“…A large variety of different magnetohydrodynamic (MHD) wave modes are theoretically predicted, and indeed observed, in solar magnetic structures ranging from large sunspots extending over several Mm, down to very small magnetic elements at the limit of spatial resolution on current solar telescopes [1][2][3][4][5][6][7][8][9][10]. Magnetic fields connect different heights in the solar atmosphere, thus MHD waves propagating along them [11] can play a major role in supplying the energy budget to the upper layers of the solar atmosphere [12][13][14][15][16][17]. When particular magnetic field geometries and local physical conditions are met, propagating MHD waves of one type can be converted into another (e.g.…”