Oscillations Above Sunspots and Faculae: Height Stratification and Relation to Coronal Fan Structure

Abstract

Oscillation properties in two sunspots and two facular regions are studied using Solar Dynamics Observatory (SDO) data and groundbased observations in the Si i 10827Å and He i 10830Å lines. The aim is to study different-frequency spatial distribution characteristics above sunspots and faculae and their dependence on magnetic-field features and to detect the oscillations that reach the corona from the deep photosphere most effectively. We used Fast-Fourier-Transform and frequency filtration of the intensity and Doppler-velocity variations with Morlet wavelet to trace the wave propagating from the photosphere to the chromosphere and corona. Spatial distribution of lowfrequency (1 -2 mHz) oscillations outlines well the fan-loop structures in the corona (the Fe ix 171 Å line) above sunspots and faculae. Highfrequency oscillations (5 -7 mHz) are concentrated in fragments inside the photospheric umbra boundaries and close to facular-region centers. This implies that the upper parts of most coronal loops, which transfer low-frequency oscillations from the photosphere, sit in the Fe ix 171 Å line-formation layer. We used dominant frequency vs. distance from barycenter relations to estimate magnetic-tube inclination angle in the higher layers, which poses difficulties for direct magnetic-field measurements. According to our calculations, this angle is ≈40°in the transition region around umbra borders. Phase velocities measured in the coronal loops' upper parts in the Fe ix 171 Å line-formation layer reach 100 -150 km s −1 for sunspots and 50 -100 km s −1 for faculae.

An Inside Look at Sunspot Oscillations with Higher Azimuthal Wavenumbers

“…For the purposes of that work, no filtering was performed in the spatial domain. However, examining time-lapse movies of the temporally filtered Hα images reveals a plethora of dynamic wave activity across a variety of spatial scales, particularly within the umbra where a dominant periodicity of ∼180 s was uncovered, which is consistent with the work of Kobanov et al (2011Kobanov et al ( , 2013Kobanov et al ( , 2015. The enhanced oscillations, which are clearly observed in the temporally (150 -180 s) filtered Hα observations, are similar in magnitude to the outputs of chromospheric umbral resonance models put forward by Zhugzhda & Locans (1981) and Staude et al (1985), whereby the upwardly propagating slow magneto-acoustic waves, which do not violate the acoustic cut-off period (e.g., Bel & Leroy 1977;Fleck & Schmitz 1991;Zhugzhda 2008;Yuan et al 2014b;Snow et al 2015, to name but a few), are reflected continuously between the steep temperature gradients present close to the photospheric temperature minimum and at the transition region boundary.…”

Long-period quasi-periodic oscillations of a small-scale magnetic structure on the Sun

“…The detected spatial and temporal distributions of the oscillations were proposed by the authors to be prescribed by the magnetic field lines topology in facular regions. Similar periodicities and the corresponding dependence of the oscillation period upon the magnetic properties of a facular region were found in Kobanov et al (2015). Freij et al (2016) detected oscillations with periods from 3 to 20 min in the intensity of two isolated magnetic pores and interpreted them in terms of a slow magnetoacoustic sausage wave.…”

Influence of the Magnetic Field on Oscillation Spectra in Solar Faculae

“…Previously we noted that low frequency oscillations dominate above faculae in the lower corona observed in the Fe ix 171 Å line (Kobanov, Kolobov, and Chelpanov, 2015;Kobanov and Chelpanov, 2014). The spatial distributions of the low-frequency oscillation power outline loop and fan structures seen in the coronal emission lines (Figure 8).…”