The relationship between the 5-min oscillation and 3-min oscillations at the umbral/penumbral sunspot boundary

Observations of the main sunspot of AR 11692 were carried out with the 1 m New Vacuum Solar Telescope (NVST) located on the Fuxian Solar Observatory (FSO) in H alpha on March 13, 2013. The high cadence (up to 12 s) H alpha intensity images help us to investigate the relationship between the 5-min oscillation and 3-min oscillation. It is found that running waves, periodically formed at the wave sources within umbra, propagate outward with the shape of partial arcs. The running waves run across the umbra-penumbra boundary and eventually disappear at the edge of penumbra. But there are obvious differences when we measure the period of running waves in different regions of a sunspot. The period is about 150 s when the running waves are located in umbra, which is a typical 3-min oscillation, and the period is about 300 s when the running waves are located in the penumbra, which is a typical 5-min oscillation. On the basis of time-slice images, we conclude that the waves form in the umbral region with the 5-min oscillation period, which can cause the brightness periodicity change in the umbra region with the 3-min period (in fact, is half of 5-min oscillation) and 5-min in the penumbra.