Solar limb brightening at 1.3 millimeters

Radial scans across the solar disk with the 29 arcsec half-power beam width of the Owens Valley Radio Observatory 10-m telescope provide evidence for 10 + or - 5% limb brightening in the mean quiet sun at 1.3 mm. This result is consistent with a spherically symmetric chromosphere. The solar radius at 1.3 mm was 8.6 + or - 1 arcsec larger than the optical radius, which is consistent with the expected effect of a forest of optically thick spicules viewed at the extreme limb.