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The umbral oscillations of velocity are commonly observed in the chromosphereof a sunspot. Their sources are considered to be either the external p-modedriving or the internal excitation by magnetoconvection. Even though thepossibility of the p-mode driving has been often considered, the internalexcitation has been rarely investigated. We report the identification of theoscillation patterns that may be closely related to the events of internalexcitation from the observations of velocity oscillations in the temperatureminimum region of two sunspots. The velocities were determined from thespectral data of the Fe I 5435 A line, a magnetically insensitive line, takenwith the Fast Imaging Solar Spectrograph of the 1.6 m Goode Solar Telescope atthe Big Bear Solar Observatory. As a result, we identified 4 oscillationpatterns of 2.0 * 10^3 km coherent size that were clearly identified for about7.9 minutes with oscillation amplitude of 9.3 * 10^-2 km/s. The power of theoscillations in these centers was concentrated in the 3 minute band. All theoscillation centers were located above the umbral dots undergoing noticeablemorphological and dynamical changes that may be regarded as an observablesignature of small-scale magnetoconvection inside the umbrae. Our resultssupport the notion that magnetoconvection associated with umbral dots insidesunspots can drive the 3 minute umbral oscillations.

The Observational Evidence for the Internal Excitation of Sunspot Oscillations Inferred from the Fe I 5435 A Line