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Measurements of the radius and limb brightening of the Sun provide importantinformation about the solar atmosphere structure and temperature. The solarradius increases as the observation at radio frequency decreases, indicatingthat each emission originates higher in the atmosphere. Thus, different layersof the solar atmosphere can be probed by observing at multiple wavelengths. Inthis work, we determined the average radius and limb brightening at 100, 212,230, and 405 GHz, using data from the Solar Submillimeter Telescope and ALMA'ssingle-dish observations. For the first time, limb brightening values forfrequencies of 212 and 405 GHz were estimated. At sub-THz frequencies, theobserved limb brightening may affect the solar radius measurements. We use twodifferent and well known approaches to determine the radius: the half-powermethod and the inflection-point method. We investigate how the antenna beamsize and the limb brightening level, LB, can affect the radius measurementsusing both methods. Our results showed that the inflection-point method is theleast affected by these parameters, and should thus be used for solar radiusestimates at radio wavelengths. The measured average radii are 968"~$\pm$~3"(100 GHz), 963"~$\pm$~3" (212 GHz), 963"~$\pm$~2" (230 GHz), and 963"~$\pm$~5"(405 GHz). Finally, we used forward modeling to estimate the ranges of LB ofthe solar disk resulting in 5%-19% (100 GHz), 2%-12% (212 GHz), 6%-18% (230GHz), and 3%-17% (405 GHz). Both radius and limb brightening estimates agreewith previous measurements reported in the literature.

Subterahertz Radius and Limb Brightening of the Sun Derived from SST and ALMA