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We report the discovery of a brightness enhancement in the center of a largesunspot umbra at a wavelength of 3 mm using the Atacama LargeMillimeter/sub-millimeter Array (ALMA). Sunspots are amongst the most prominentfeatures on the solar surface, but many of their aspects are surprisinglypoorly understood. We analyzed a {\lambda}=3 mm (100 GHz) mosaic image obtainedby ALMA, which includes a large sunspot within the active region AR12470 onDecember 16, 2015. The 3 mm map has a field-of-view and spatial resolution,which is the highest spatial-resolution map of an entire sunspot in thisfrequency range. We find a gradient of 3 mm brightness from a high value in theouter penumbra to a low value in the inner penumbra/outer umbra. Within theinner umbra, there is a marked increase in 3mm brightness temperature, which wecall an umbral brightness enhancement. This enhanced emission corresponds to atemperature excess of 800 K relative to the surrounding inner penumbral regionand coincides with excess brightness in the 1330 and 1400 {\AA} slitjaw imagesof the Interface Region Imaging Spectrograph (IRIS), adjacent to a partiallightbridge. This {\lambda}=3 mm brightness enhancement may be an intrinsicfeature of the sunspot umbra at chromospheric heights, such as a manifestationof umbral flashes, or it could be related to a coronal plume since thebrightness enhancement was coincident with the footpoint of a coronal loopobserved at 171 {\AA}.

ALMA Discovery of Solar Umbral Brightness Enhancement at {\lambda}=3 mm