Volume 554, June 2013
|Number of page(s)||6|
|Published online||18 June 2013|
Oscillations above sunspots from the temperature minimum to the corona
Institute of Solar-Terrestrial Physics of Siberian Branch of Russian Academy of Science, PO Box 291 Lermontov st., 126-a, Irkutsk 664033, Russia
Received: 12 October 2012
Accepted: 3 May 2013
Context. An analysis of the oscillations above sunspots was carried out using simultaneous ground-based and Solar Dynamics Observatory (SDO) observations (Si i 10827 Å, He i 10830 Å, Fe i 6173 Å, 1700 Å, He ii 304 Å, Fe ix 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 ii 304 Å and Fe ix 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 i 10827 Å–He i 10830 Å, 1700 Å–He ii 304 Å, and He ii 304 Å–Fe ix 171 Å height levels, respectively.
Key words: Sun: chromosphere / Sun: oscillations / sunspots / Sun: corona
© ESO, 2013
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