In the high layers, sunspots show oscillatory behavior; 3 min standing oscillations are dominant in the umbra, while running 5 min waves are dominant in the penumbra and superpenumbra. Running penumbral waves (RPW) were first observed in H by Zirin & Stein (1972) and independently by Giovanelli (1972, 1974). Since their discovery researchers have tried to determine their nature as well as their relation to other phenomena such as umbral oscillations and p-mode photospheric oscillations.
The horizontal propagation speed of penumbral waves is typically in the range of 10-20 km s-1. This is close to the sound speed in the chromosphere and of the Alfven speed in the photosphere but much less than the Alfven speed in the penumbral chromosphere. Thus, if the waves are not mainly compression waves, the travelling oscillation observed in the chromosphere is the vertical extension of a wave propagating in the photosphere.
A number of authors have tried to examine how deeply situated in the photosphere the waves are. Lites (1988) reports that he was able to clearly observe penumbral waves in the inner penumbra as viewed in the Fe I 543.4 nm line. Fe I 543.4 nm line is believed to be formed in the low umbral photosphere, just above the temperature minimum (Lites & Thomas, 1985). Musman et al. (1976) searched for penumbral waves in the low photosphere using simultaneous measurements in the H line and the Zeeman-insensitive line Fe I 557.6 nm. According to the above authors the Fe I 557.6 nm line is formed in the upper penumbral photosphere (between 250 and 530 km above the optical depth unity at 500 nm). They found waves propagating horizontally outwards across the penumbra with about the same period as the RPW in H. However, the waves were more intermittent and had a higher horizontal phase velocity by a factor of 2 or more than the chromospheric penumbral waves. Thus, the connection between the photospheric and chromospheric waves is not clear. Lites et al. (1982) were not able to find obvious penumbral disturbances in the Fe I 5576 intensity or Doppler images.
From the above analysis the conclusion is that there is not a clear picture concerning the waves observed at the photospheric level. In a previous paper (Christopoulou et al. 2000, referred to as Paper I from now on) we have studied umbral oscillations, running penumbral waves and the relationship between them in the Chromospheric level. In this paper, we present waves related to sunspots observed in the Fe I 557.6 nm line.
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000