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

Dmitri Kolobov   Submitted: 2014-12-22 03:28

Oscillation properties in two sunspots and two facular regions are studied using Solar Dynamics Observatory (SDO) data and ground-based observations in the SiI 10827 and HeI 10830 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 low-frequency (1-2 mHz) oscillations outlines well the fan-loop structures in the corona (the Fe IX 171 line) above sunspots and faculae. High-frequency 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 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 about 40 degrees in the transition region around umbra borders. Phase velocities measured in the coronal loops' upper parts in the Fe IX 171 line-formation layer reach 100-150 km s-1 for sunspots and 50-100 km s-1 for faculae.

Authors: N.I. Kobanov, D.Y. Kolobov, A.A. Chelpanov
Projects: SDO-AIA,SDO-HMI

Publication Status: published in Solar Physics https://dx.doi.org/10.1007/s11207-014-0623-6
Last Modified: 2014-12-22 08:03
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Direct measurement results of the time lag of LOS-velocity oscillations between two heights in solar faculae and sunspots  

Dmitri Kolobov   Submitted: 2013-06-30 03:07

We present an investigation of line-of-sight (LOS) velocity oscillations in solar faculae and sunspots. To study the phase relations between chromosphere and photosphere oscillations of the LOS velocity, we measured the time lag of the chromospheric signal relative to the photospheric one for several faculae and sunspots in a set of spectral line pairs. The measured time lags are different for different objects. The mean measured delay between the oscillations in the five-minute band in faculae is 50s for the SiI 10827{AA}-HeI 10830{AA} pair; for the pair FeI 6569{AA}-Hα 6563{AA} the mean delay is 20s; for the pair FeI 4551{AA}-BaII 4554{AA} the mean delay is 7s; for the pair SiI 8536{AA}-CaII 8542{AA} the mean delay is 20s. For the oscillations in the three-minute band in sunspot umbrae the mean delay is 55s for the SiI 10827{AA}-HeI 10830{AA} pair; for the Fe I 6569{AA}-Hα 6563{AA} pair it was not possible to determine the delay; for the FeI 4551{AA}-BaII 4554{AA} pair the mean delay is 6s; for the SiI 8536{AA}-CaII 8542{AA} pair the mean delay is 21s. Measured delays correspond to the wave propagation speed which significantly exceeds the generally adopted speed of sound in the photosphere. This raises the question of the origin of these oscillations. The possibility that we deal with slow MHD waves is not ruled out.

Authors: N.I. Kobanov, D.Y. Kolobov, A.S. Kustov, S.A. Chupin, A.A. Chelpanov
Projects: Other

Publication Status: Solar Physics, 2013, Volume 284, Issue 2, pp 379-396
Last Modified: 2013-07-01 09:06
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Oscillations above sunspots from the temperature minimum to the corona  

Dmitri Kolobov   Submitted: 2013-06-30 03:03

Context. An analysis of the oscillations above sunspots was carried out using simultaneous ground-based and Solar Dynamics Observatory (SDO) observations (SiI 10827A, HeI 10830A, FeI 6173A, 1700A, HeII 304A, FeIX 171A). 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 A), 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 (HeII 304A and FeIX 171A). 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 SiI 10827A-HeI 10830A, 1700A-HeII 304A, and HeII 304A-FeIX 171A height levels, respectively.

Authors: N. I. Kobanov, A. A. Chelpanov, and D. Y. Kolobov
Projects: Other,SDO-AIA,SDO-HMI

Publication Status: published in A&A 554, A146 (2013)
Last Modified: 2013-07-01 09:06
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Abstracts by Author
Oscillations Above Sunspots and Faculae: Height Stratification and Relation to Coronal Fan Structure
Direct measurement results of the time lag of LOS-velocity oscillations between two heights in solar faculae and sunspots
Oscillations above sunspots from the temperature minimum to the corona

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