Photometry of a solar flare.

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Abstract
Using the direct intensity recording microdensitometer at the University of Michigan I have made isophotes of a solar flare. Five frames of the Ha record taken at the McMath Hulbert Observatory of the flare of July 22, 1950 were selected for the analysis. I found it necessary to print each of these frames and the standardization wedge on lantern slide plates in order to trace them in the microdensitometer. The standardization wedge on each plate was compared with the working density wedge of the microdensitometer. The chemical fog on each plate was reduced to the fog on the working wedge. The isophotes correspond to intensities in the flares such that log 1b = 0.05, 0.10, 0.15, 0.20. Here I is the intensity at a point in the flare and 1b is the average intensity of the photosphere in each frame. The isophotes show that the intensity gradient in the flare, during its maximum stages, is ex- ~Now at the Yerkes Observatory. tremely high. This confirms the observation of the visual workers that flares have a "hard outline." This steep gradient disappears in the post maximum stages of the flare. One also finds that the bright core of the flare is extremely small at the maximum. I am deeply indebted to Dr. H. Dodson and Mr. E. Dennison for their aid in this work. University of Michigan Observatory, Ann Arbor, Mich.

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