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Astron. Astrophys. 363, 306-310 (2000)

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4. Discussion and conclusions

A number of authors proposed that p-mode oscillations play a fundamental role in the generation of at least part of the oscillations and waves that we observe in sunspots. Abdelatif & Thomas (1987) examined the interaction of solar p-modes with a sunspot. In their treatment they considered the sunspot as a magnetic slab where incident acoustic wave packets from the surrounding photosphere are partially reflected and partially transmitted into the slab.

Lites (1988) observed in Fe I 543.4 nm Doppler amplitude images, a ring of low oscillatory amplitude halfway between the inner and outer edge of the penumbra. He interpreted this ring as a result of the p-mode oscillations becoming increasingly more important in the outer penumbra of sunspots. He also reports that although he was able to clearly observe penumbral waves in the inner penumbra, the p-modes encroaches over the outer penumbra. The low oscillatory amplitude ring he observed seems to be related to the dividing line of Sobotka et al. (1999) and our splitting line where the waves start moving towards opposite directions.

It is possible that p-mode oscillations play a fundamental role in the generation of the waves we observed. Wave packets transmitted into the penumbra would slowly travel towards the umbra. Reflected wave packets would travel outwards in the area around the sunspot. The propagation phase velocity of the waves is very small and does not correspond to the photospheric sound velocity. A possible scenario is that the wave we observe is the result of the superposition of two different waves moving in opposite directions. The result is a nearly standing wave slowly moving towards the umbra in case of the penumbra and outwardly in case of the area around the sunspot. This scenario is enhanced by the fact that the horizontal wavelength of the photospheric waves is about half that of the chromospheric ones, as well as the coherent behavior of the 5-min oscillations around the spot. The power of the waves due to the p-mode oscillations is diminished as we move through higher layers. In the Fe I 543.4 nm line we observe the behavior referred to by Lites (1988), and in the chromosphere we observe waves starting out from the umbra and propagating outwards.

Another possible scenario is that the waves are related to oscillations in subphotospheric layers due to convection. Galloway (1978) modeled the formation of a sunspot by the action of convection on a weak background magnetic field. He found solutions in which steady convection concentrates the field into a strong central rope and subsequently oscillations develop within the rope. Recently Hurlburt & Rucklidge (2000) made numerical experiments modelling the interaction of pores and sunspots with solar convection. They proposed that except for the outflow around sunspots, there is an inflow hidden beneath the penumbrae of large sunspots (cf. their Fig. 8). In this case it is possible that convection drives both the waves we observe and the motions of penumbral grains.

More theoretical analysis and further observations are needed to exploit all possible scenarios. Further analysis is also needed for the clarification of the relation of the photospheric waves with the waves and oscillations observed in the chromospheric level. This will be done based on a new run at the DST.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000