A first look at the sub-mm Sun with ALMA

We present the first full-disk solar images obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 7 (0.86 mm; 347 GHz). In spite of the low spatial resolution (21 arcsec), several interesting results were obtained. During our observation the sun was practically devoid of active regions. Quiet Sun structures on the disk are similar to those in Atmospheric Imaging Assembly (AIA) images at 1600A and 304A, after the latter are smoothed to the ALMA resolution, as noted previously for Band 6 (1.26 mm) and Band 3 (3 mm) images; they are also similar to negative H-alpha images of equivalent resolution. Polar coronal holes, well seen in the 304A band, are not detectable at 0.86 mm. We computed the center-to-limb variation (CLV) of the brightness temperature in Band 7, as well as in Bands 6 and 3, obtained during the same campaign, and combined them to a unique curve of T_b(log μ₁₀₀), where μ₁₀₀ is the cosine of the heliocentric angle reduced to 100 GHz. Assuming that the absolute calibration of the Band 3

commissioning observations is accurate, we deduced a quiet Sun brightness temperature at the center of the disk of 6085 K for Band 7, instead of the value of 5500 K, extrapolated from the recommended values for bands 3 and 6. More important, the T_bb(log μ₁₀₀) curve flattens at large values of μ₁₀₀, and so does the corresponding T_e(log τ₁₀₀) at large τ₁₀₀. This is probably an indication that we are approaching the temperature minimum.