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We investigate the upper chromosphere and the transition region of thesunspot umbra using the radio brightness temperature at 34 GHz (correspondingto 8.8-mm observations) as observed by the Nobeyama Radioheliograph (NoRH).Radio free-free emission in the longer millimeter range is generated around thetransition region, and its brightness temperature yields the region'stemperature and density distribution. We use the NoRH data at 34 GHz byapplying the Steer-CLEAN image synthesis. These data and the analysis methodenable us to investigate the chromospheric structures in the longer millimeterrange with high spatial resolution and sufficient visibilities. We also performsimultaneous observations of one sunspot using the NoRH and the Nobeyama 45-mtelescope operating at 115 GHz. We determine that 115-GHz emission mainlyoriginates from the lower chromosphere while 34-GHz emission mainly originatesfrom the upper chromosphere and transition region. These observational resultsare consistent with the radio emission characteristics estimated from thecurrent atmospheric models of the chromosphere. On the other hand, the observedbrightness temperature of the umbral region is almost the same as that of thequiet region. This result is inconsistent with the current sunspot models,which predict a considerably higher brightness temperature of the sunspot umbraat 34 GHz. This inconsistency suggests that the temperature of the region atwhich the 34 GHz radio emission becomes optically thick should be lower thanthat predicted by the models.

Chromospheric sunspots in the millimeter range as observed by the Nobeyama Radioheliograph