  Helioseismic and magnetohydrodynamic waves are abundant in and above
sunspots. Through cross-correlating oscillation signals in the photosphere
observed by the SDO/HMI, we reconstruct how waves propagate away from virtual
wave sources located inside a sunspot. In addition to the usual helioseismic
wave, a fast-moving wave is detected traveling along the sunspot's radial
direction from the umbra to about 15 Mm beyond the sunspot boundary. The wave
has a frequency range of 2.5 - 4.0 mHz with a phase velocity of 45.3 km/s,
substantially faster than the typical speeds of Alfven and magnetoacoustic
waves in the photosphere. The observed phenomenon is consistent with a scenario
of that a magnetoacoustic wave is excited at approximately 5 Mm beneath the
sunspot, and its wavefront travels to and sweeps across the photosphere with a
speed higher than the local magnetoacoustic speed. The fast-moving wave, if
truly excited beneath the sunspot's surface, will help open a new window to
study the internal structure and dynamics of sunspots.

Solar and Stellar Astrophysics