  Polar brightening of the Sun at radio frequencies has been studied for almost
fifty years and yet a disagreement persists between solar atmospheric models
and observations. Some observations reported brightening values much smaller
than the expected values obtained from the models, with discrepancies being
particularly large at millimeter wavelengths. New clues to calibrate the
atmospheric models can be obtained with the advent of the Atacama Large
Millimeter/submillimeter Array (ALMA) radio interferometer. In this work, we
analyzed the lower limit of the polar brightening observed at 100 and 230 GHz
by ALMA, during its Science Verification period, 2015 December 16-20. We find
that the average polar intensity is higher than the disk intensity at 100 and
230 GHz, with larger brightness intensities at the South pole in eight of the
nine maps analyzed. The observational results were compared with calculations
of the millimetric limb brightnening emission for two semi-empirical
atmospheric models, FAL- C (Fontenla et al. 1993) and SSC (Selhorst et al.
2005a). Both models presented larger limb intensities than the averaged
observed values. The intensities obtained with the SSC model were closer to the
observations, with polar brightenings of 10.5% and 17.8% at 100 and 230 GHz,
respectively. This discrepancy may be due to the presence of chromospheric
features (like spicules) at regions close to the limb.
