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Velocity and intensity oscillations in sunspot penumbrae
M. Sigwarth and
Received 5 August 1996 / Accepted 2 December 1996
Velocity and intensity oscillations in the penumbra of a sunspot close to disc center are investigated at various atmospheric heights from the photosphere to the upper chromosphere using simultaneous time series of spectra of FeI 5123 Å, Mg b1, Na D1/D2, NiI 5893 Å, H and CaII 8542 Å. Line core shifts and intensity variations are determined for all positions along the slit and power, coherence and phase spectra are calculated.
Both Doppler velocities and line core intensity fluctuations are significantly reduced inside the penumbra and the surrounding plage region in all atmospheric layers compared to the surrounding quiet sun. Inside the penumbra the velocity oscillations are found to be coherent over a larger spatial range in photospheric and middle chromospheric layers than outside.
Velocity power spectra show the 5-minute oscillations up to lower chromospheric levels from the inner to outer penumbra boundary. Only in the layer below the temperature minimum the 5-minute oscillations are not detectable in the inner and central penumbra.
In the upper chromosphere frequencies significantly changes from 6 mHz at the inner penumbra to around 1.5 mHz at the outer penumbra boundary.
Intensity oscillations show power at low frequencies ( 2 mHz) outside the spot and in the outer penumbra in all atmospheric layers while in the central penumbra power in the 5-minute range is present only in the layer below the temperature minimum.
From a coherence and phase analysis we find upward propagating waves inside the penumbra with a different behavior in deeper and higher layers and compared to the quiet sun.
No differences could be found for the oscillatory behavior in dark and bright penumbral filaments (at a mean spatial resolution of 1 arcsec).
Key words: Sunspots oscillations photosphere chromosphere
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© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998