Oscillations above sunspots from the temperature minimum to the corona

Abstract

Context. An analysis of the oscillations above sunspots was carried out using simultaneous ground-based and Solar Dynamics Observatory (SDO) observations (Si  10827 Å, He  10830 Å, Fe  6173 Å, 1700 Å, He  304 Å, Fe  171 Å). Aims. Investigation of the spatial distribution of oscillation power in the frequency range 1-8 mHz for the different height levels of the solar atmosphere. Measuring the time lags between the oscillations at the different layers. Methods. We used frequency filtration of the intensity and Doppler velocity variations with Morlet wavelet to trace the wave propagation from the photosphere to the chromosphere and the corona. Results. The 15 min oscillations are concentrated near the outer penumbra in the upper photosphere (1700 Å), forming a ring that expands in the transition zone. These oscillations propagate upward and reach the corona level, where their spatial distribution resembles a fan structure. The spatial distribution of the 5 min oscillation power looks like a circle-shape structure matching the sunspot umbra border at the photospheric level. The circle expands at the higher levels (He  304 Å and Fe  171 Å). This indicates that the low-frequency oscillations propagate along the inclined magnetic tubes in the spot. We found that the inclination of the tubes reaches 50−60 degrees in the upper chromosphere and the transition zone. The main oscillation power in the 5-8 mHz range concentrates within the umbra boundaries at all the levels. The highest frequency oscillations (8 mHz) are located in the peculiar points inside the umbra. These points probably coincide with umbral dots. We deduced the propagation velocities to be 28 ± 15 km s −1 , 26 ± 15 km s −1 , and 55 ± 10 km s −1 for the Si  10827 Å-He  10830 Å, 1700 Å-He  304 Å, and He  304 Å-Fe  171 Å height levels, respectively.

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.…”

Oscillations and Waves in Sunspots

“…Based on the intrinsic relationships they find between the underlying magnetic field geometries connecting the photosphere to the chromosphere, and the characteristics of running penumbral waves observed in the upper chromosphere, and Jess et al (2013) conclude that running penumbral wave phenomena are the chromospheric signature of upwardly propagating magneto-acoustic waves generated in the photosphere. Kobanov et al (2013a) obtained similar results for the wave power distribution at different frequencies and heights. In addition, the latter authors measured the time lag between oscillations at different levels.…”

“…This argument has been used to suggest that running penumbral waves are not associated with similar waves in the umbra and that very likely umbral and penumbral oscillations initially propagate along different magnetic field lines. Through careful consideration of the magnetic vector, Bloomfield et al (2007a) provided evidence that velocity signatures of running penumbral waves observed in the He i 10830 multiplet are more compatible with upwardpropagating waves than with trans-sunspot waves, see Section 8. , , Jess et al (2013), and Kobanov et al (2013a) have investigated the role of the magnetic field topology in the propagation characteristics of umbral and running penumbral waves at chromospheric, transition region and coronal layers. They find an increase of the oscillatory period of brightness oscillations as a function of distance from the umbral center.…”

Spatial distributions of sunspot oscillation modes at different temperatures

“…High spatial and temporal resolution observations provided a chance to reveal spatial distributions of sunspot oscillations in different atmospheric layers. Using data observed by Sayan Solar Observatory and Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA; Lemen et al 2012;Pesnell et al 2012), Kobanov et al (2013) found that the powers of the three-minute oscillation mainly concentrate within an umbra in the temperature minimum and chromosphere. Although the oscillation expands significantly in the transition region and lower corona, it is still limited within the umbra.…”