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A.A. Georgakilas (Caltech), E.B. Christopoulou (Un. Patras), H. Zirin (Caltech)
Sunspots show oscillatory behavior, identifiable as intensity and velocity variations in photospheric and chromospheric lines. In the chromospheric layers 3 min standing oscillations are dominant in the umbra (umbral oscillations), while 5 min running waves are dominant in the penumbra and superpenumbra. Penumbral waves (RPW) were first observed in H\alpha by Zirin and Stein (1972) and independently by Giovanelli (1972). Since their discovery a number of authors have tried to determine the physical properties of umbral oscillations and running penumbral waves (like the frequency and the propagation velocity) and to identify their nature, as well as the relation between them. In order to clarify discrepancies, due to the analysis of limited number of sunspots, we present comprehensive results from the study of 8 sunspots observed from the Big Bear Solar Observatory (BBSO) during 1999 and 2000. Our results show that RPW are propagating with a mean velocity of the order of 15 km~s-1. The fourier analysis of 'time slice images' (created taking cross sections of every image of a time series) indicates that there is not a clear relationship between umbral oscillations and running penumbral waves.
This work was supported by NSF grant ATM-9726147.
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