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Oscillations and running waves observed in sunspots
E.B. Christopoulou 1,
A.A. Georgakilas 1 and
S. Koutchmy 2
Received 18 October 1999 / Accepted 1 December 1999
In order to study umbral oscillations, running penumbral waves and the relationship between them, we analyzed CCD, high resolution, sunspot observations obtained at center and the wings of the H line and the Fe I 5576 Å line. The UBF filter was used in order to produce high cadence sequences of filtergrams. Images were processed to remove the sharp intensity gradient between the umbra and the penumbra. They show the waves to start out around umbral oscillating elements and to propagate outwards forming concentric cycles around the elements. The waves appear to propagate beyond the outer edge of the photospheric penumbra, in the superpenumbra, where they dilute. Comparing images in 9 wavelengths along the H profile we found out that the waves are definitely better observed near the H center and near the blue wing -0.35 Å. This indicates a possible vertical upward mass motion in the oscillating penumbral structure and that the oscillation is not symmetric about zero.
We found different oscillating modes. Standing umbral oscillations are dominant in the umbra and inner penumbra; their frequency is around 6.5 mHz. Similar oscillations are observed in the penumbra - superpenumbra boundary but with considerably lower frequency (2 mHz). Oscillations are absent or have reduced magnitude in the central part of the penumbra. Penumbral waves are running waves propagating with a constant phase velocity around 13 km s-1; their frequency is remaining constant over the whole penumbra, in the band of 3 mHz. We produced "time slice images" which show, that there is not a clear relationship between umbral oscillations and running penumbral waves.
Key words: Sun: sunspots Sun: oscillations
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