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The subsurface structure of a solar sunspot is important in the stability ofthe sunspots and the energy transport therein. Two subsurface structure modelshave been proposed, the monolithic and cluster models, but no clearobservational evidence supporting a particular model has been found so far. Toobtain clues about the subsurface structure of sunspots, we analyzed umbralflashes in merging sunspots registered by IRIS Mg II 2796 Angstrom slit-jawimages (SJIs). Umbral flashes are regarded as an observational manifestation ofmagnetohydrodynamic (MHD) shock waves originating from convection cells belowthe photosphere. By tracking the motion of individual umbral flashes, wedetermined the position of the convection cells that are the oscillationcenters located below the umbra. We found that the oscillation centers arepreferentially located at dark nuclei in the umbral cores rather than in brightregions such as light bridges or umbral dots. Moreover, the oscillation centerstend to deviate from the convergent interface of the merging sunspots wherevigorous convection is expected to occur. We also found that the inferreddepths of the convection cells have no noticeable regional dependence. Theseresults suggest that the subsurface of the umbra is an environment whereconvection can occur more easily than the convergent interface, and hencesupport the cluster model. For more concrete results, further studies based onumbral velocity oscillations in the lower atmosphere are required.

Investigation of the subsurface structure of a sunspot based on the spatial distribution of oscillation centers inferred from umbral flashes