Measurement of the solar limb brightness profile at 3 millimeters during the total eclipse of 1991 July 11

We observed the solar limb at the point of first contact during the eclipse of 1991 July 11, with a spatial resolution ∼1″.6. The observations were carried out at the Owens Valley Radio Observatory millimeter interferometer, operating at 3 mm (99.46 GHz, with a bandwidth of 350 MHz). The visibility amplitude and phase were modeled to yield the height of the 3 mm limb above the visible photosphere, and the data were differentiated to yield the brightness profile of the limb in strips ∼1″.6 wide. We found that the 3 mm limb extends 7″.5 ± 0″.8 or 5500 km above the visible photosphere, with no evidence of a limb spike. This result, and the overall shape of the limb profile, are similar to the interferometric results of Wannier, Hurford, & Seielstad (1983) obtained with the same instrument but without the benefit of an eclipse. The 3 mm limb, at a temperature of ∼6500 K, extends to altitudes far beyond the expected location of the transition region in the model of Vernazza, Avrett, & Loeser (1981). A comparison of the 3 mm profile and an off-band Hα photograph of the limb reveals a close correspondence between the 3 mm limb and the height of Hα spicules.