Center-to-limb observations of the Sun with ALMA

Abstract

We measured the center-to-limb variation of the brightness temperature, $T_b$, from ALMA full-disk images at two frequencies and inverted the solution of the transfer equation to obtain the electron temperature, $T_e$ as a function of optical depth, $\tau$. The ALMA images are very similar to AIA images at 1600Å. The brightness temperature at the center of the disk is 6180 and 7250 K at 239 and 100 GHz respectively, with dispersions of 100 and 170 K. Plage regions stand out clearly in the 239/100 GHz intensity ratio, while faculae and filament lanes do not. The solar disk radius, reduced to 1 AU, is $961.1\pm 2.5$ arcsec and $964.1\pm 4.5$ arcsec at 239 and 100 GHz respectively. A slight but statistically significant limb brightening is observed at both frequencies. The inversion of the center-to-limb curves shows that $T_e$ varies linearly with the logarithm of optical depth for $0.34<{\tau }_{100GHz}<12$, with a slope $d\ln T_e/d\tau =-608$ K. Our results are 5% lower than predicted by the average quiet sun model C of Fontenla et al. (1993), but do not confirm previous reports that the mm-$\lambda$ solar spectrum is better fitted with models of the cell interior.

Accepted for publication in Astronomy and Astrophysics

References in corpus (4)

• Observing the Sun with the Atacama Large Millimeter-submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping
• The Brightness Temperature of the Quiet Solar Chromosphere at 2.6 mm
• Observation of Chromospheric Sunspot at Millimeter Range with the Nobeyama 45 m Telescope
• RATAN-600 Observations of Small Scale Structures with High Spectral Resolution

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