Running waves observed by Hinode

High cadence (up to 9.6 s) intensity images of a sunspot within the active region NOAA 10953 were obtained by the SOT onboard Hinode with the line Hα 6563 Å on April 30, 2007. The running difference images show that running waves periodically appear within the sunspot umbra and propagate outward forming full circles or partial arcs. The propagation velocity of the running waves decreases progressively when these waves run away from the umbra center. A special relationship between the propagation velocity v and the wavefront radius r of the running waves, v=kr+r0, implies that the running waves in Hα line may be fast magnetoacoustic waves. By using FFT, we determined the period of running waves as being approximately 275 s.