Visual and Spectrographic Observations of a Great Solar Flare, 1946 July 25

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Abstract
Measures of effective line-width of the bright Hα emission support previous results in bringing to light the existence of what may be described as a radiation burst at the commencement of a great flare. Its duration is of order of 2–3 minutes only, as compared with a total duration for the flare of rseveral hours. Seven flare spectra were obtained, covering the period before and after the time of maximum intensity at 16 h 27 m . One plate, exposed within one minute of the time of peak intensity, is of special interest, and the following unusual features have been recorded : ( a ) Reversal of the Hα line 15 A . wide, with a central intensity greater than 300 per cent, of the continuous spectrum of the photosphere. The wings of the bright emission show an asymmetry which has been noted at Sherborne in all previous flares. ( b ) Helium λ 6678 in strong emission. Helium λ 5876 in emission over the flare and in absorption over the penumbra of the sunspot as well as over the surrounding photosphere. ( c ) Reversals of λλ 6347.1, 6371.4 of ionized silicon. ( d ) Contours of many iron and nickel lines filled in. ( e ) An enhancement of the continuous spectrum over a small area which coincides in position with the brightest part of the flare filament as seen by light of Hα . This phenomenon appears on only one plate and is simultaneous with the radiation burst. It seems probable that we have here the explanation of the phenomenon observed by Carrington and Hodgson in integrated light on 1859 September 1. These observations all point to the need for continuous spectrographic records at short intervals, especially during the first few minutes of flare activity.

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