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A Peculiar Velocity Pattern in and near the Leading Sunspot of NOAA 10781: Wave Refraction by Large-Scale Magnetic Fields?

I report observations of unusually strong photospheric and chromospheric velocity oscillations in and near the leading sunspot of NOAA 10781 on 03 July 2005. I investigate an impinging wave as a possible origin of the velocity pattern, and the changes of the wave after the passage through the magnetic fields of the sunspot. The wave pattern found consists of a wave with about 3 Mm apparent wavelength that propagates towards the sunspot. This wave seems to trigger oscillations inside the sunspot's umbra, which originate from a location inside the penumbra on the side of the impinging wave. The wavelength decreases and the velocity amplitude increases by an order of magnitude in the chromospheric layers inside the sunspot. On the side of the sunspot opposite to the impinging plane wave, circular wave fronts centered on the umbra are seen propagating away from the sunspot outside its outer white-light boundary. They lead to a peculiar ring structure around the sunspot, which is visible in both velocity and intensity maps. The fact that only weak photospheric velocity oscillations are seen in the umbra - contrary to the chromosphere where they peak there - highlights the necessity to include the upper solar atmosphere in calculations of wave propagation through spatially and vertically extended magnetic field concentrations like sunspots.