Oscillations Above Sunspots and Faculae: Height Stratification and Relation to Coronal Fan Structure
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Oscillation properties in two sunspots and two facular regions are studied using Solar Dynamics Observatory (SDO) data and ground-based observations in the Si i 10827Å and He i 10830 Å 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 a Morlet wavelet to trace the waves that propagate from the photosphere to the chromosphere and corona. The spatial distribution of low-frequency (1 – 2 mHz) oscillations clearly outlines the fan-loop structures in the corona (the Fe ix 171 Å line) above sunspots and faculae. High-frequency oscillations (5 – 7 mHz) are concentrated in fragments inside boundaries of the photospheric umbra and close to facular-region centers. This implies that the upper parts of most coronal loops, which transfer low-frequency oscillations from the photosphere, are located in the Fe ix 171 Å line-formation layer. We used the relations of the dominant frequency vs. distance from barycenter to estimate the 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 the umbra borders. Phase velocities measured in the coronal loop 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.
KeywordsSunspots, Oscillations Faculae, Oscillations Magnetic topology Coronal loops
The study was performed with partial support of Projects No. 16.3.2, 16.3.3 of ISTP SB RAS. We acknowledge E. Korzhova for her help in preparing the English version of the article and the NASA/SDO science team for providing the data. We are grateful to an anonymous referee for helpful remarks and suggestions.
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