High-resolution observations revealed widespread oscillations and waves in sunspots. However, it is still unclear whether these oscillations and waves are driven by the acoustic waves (p-modes) or generated by the magnetoconvection. To obtain clues about the driving source of umbral waves in sunspots, We investigated the spiral wave patterns (SWPs) in two sunspots based on IRIS SJI 2796 Å imaging data. By tracking the motion of the spiral wave patterns, we find that two one-armed spiral wave patterns coexist in the umbra, and they can rotate either in the same or opposite directions. Furthermore, by statistically analyzing the spatial distribution of the oscillation centers of the one-armed spiral wave patterns within the umbra (the oscillation center is defined as the location where the spiral wave pattern first appears), we find that the chromospheric umbral waves repeatedly originate from the regions with high oscillation power and most of the umbral waves occur in the dark nuclei and strong magnetic field regions of the umbra, rather than in those regions outside the dark nuclei where magnetoconvection is stronger. Therefore, our study results are more inclined to support the p-modes rather than the magnetoconvection driving umbral waves.
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