Color photometry of the solar corona on July 31, 1981

Photographs of the outer solar corona obtained by the Soviet-French expedition to Kazakhstan during the total eclipse of July 31, 1981, are investigated. An F = 300 mm/5.6 camera and 60-millimeter color reversal Ektachrome-64 film were used. The radial brightness distribution is obtained along radial cross sections passing through the south 'coronal hole' and a bright region of the corona in blue (450 nm) and red (630 nm) light. By comparison with the sun, the corona is redder (by at least 10 percent) at distances of 2-6 solar radii. Allowance is made for the emission of the sky and the aureole. Isophotes are constructed (r less than 10 solar radii) in blue and red light. The brightness of the corona averaged over all directions also points to reddening. The flattening of the outer isophotes increases with distance.