In order to study running waves we produced movies in Fe I +0.012 nm and Fe I -0.012 nm, as well as Doppler velocity movies in Fe I 0.012 nm. Since the waves seem to be very weak it was very difficult to identify and follow them although in some of the best images they are very clear (Fig. 1). In order to better identify the waves and measure their propagation velocity, we followed the methods described in the previous section in order to create "time slice images". Fig. 2 shows "time slice images" in Fe I -0.012 nm, computed using method I. Fig. 3, Fig. 4, Fig. 5 and Fig. 6 show "time slice images" in Fe I -0.012 nm, Fe I +0.012 nm, Doppler images and sum images (Fe I 0.012 nm) computed using method II. The integration angle was 45 o. We used the region in the white frame in Fig. 1. The waves are clearly identifiable in these images: they appear as diagonal streaks, curving forward in time. Comparing Fig. 2 and Fig. 3, we verify that method II gives better results.
We observe slow waves both in the penumbra and the area around the sunspot corresponding to the chromospheric superpenumbra. The waves appear to begin at the outer part of the penumbra. They are directed inwards in the penumbra (towards the umbra) and outwards in the area around the penumbra. Their propagation velocity is near 0.5 km s-1 in both cases and their horizontal wavelengths near 2300 km. We should mention that the horizontal wavelength of the waves observed in the chromosphere for the same sunspot is about 4500 km. The waves clearly appear both in the blue and red wings, and more importantly in the sum of the wings, suggesting that they are not related to velocity perturbations but probably to pressure or temperature fluctuations.
Beside the waves, high frequency oscillations are observed in the time slice images (12 and 15 MHz); they should not be of Solar origin. Lites et al. (1982) analyzing similar observations in the Fe I 557.6 nm line also observed such high frequency oscillations and they related the phenomenon to their sampling rate.
The slow waves should not be related to the Evershed phenomenon since in the photosphere the Evershed flow is directed outwards and is restricted to the penumbra. Muller (1973), from high resolution observations, reported that the penumbra appeared to consist of bright grains moving towards the umbra of the spot. He found that the horizontal velocity of the grains was zero at the border of the penumbra-photosphere and maximum at the umbral border (0.5 km s-1). Sobotka et al. (1999) made a similar analysis and found that there appeared to be a dividing line (DL) in the penumbra, at approximately 0.7 of the distance from the umbra to the photosphere; most penumbral grains outside this line moved toward the photosphere and those inside moved toward the umbra. For inward-moving grains they found a typical proper motion speed of 0.4 km s-1 and for the outward moving ones, 0.5 km s-1. Their results are similar to ours concerning the dividing line and the velocity of the grains compared to that of the waves. However, we should note that in our case we refer not to grains but to waves forming arcs around the spot, as is obvious in Fig. 1.
We should further note that the phase propagation velocity of the outward moving waves is similar to the velocity of the moat flows as well as that of Moving Magnetic Features (MMFs), first observed three decades ago on sequences of magnetograms (see Harvey & Harvey, 1973). Finally, let we note that recently Rast et al. (1999) observed systematic bright rings around twelve isolated sunspots, within one sunspot radius of the penumbra.
It is interesting that in Fig. 5 we observe five minute oscillations around the spot, although we have integrated along a 45 o angle; the oscillations are clear even if we integrate along 90 o or 180 o (Fig. 7). This implies a coherent behavior of the five-minute oscillations around the spot .
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000