Large Sunspot Group of February 1935

Abstract

ALTHOUGH the sunspot group referred to in NATURE of February 16 (p. 260) was not specially large, it was of interest because of its very rapid growth. Spectro-heliograms in K_{2, 3, 2} light were secured through thin cloud at the Solar Physics Observatory, Cambridge, on February 5, 6 and 7. On February 5 the group is shown on a plate exposed from 12^h 25^m 18^s to 12^h 26^m 51^s, and as the formation was near the centre of the disc, it must have been recorded very near 12^h 26^m 5^s. As no spot was shown on the Greenwich photograph at 10^h, this fixes the time of formation of the spot to within 2½ hours. On the spectroheliogram the feature was very small. On February 6 there is evidence of very rapid growth ; preceding and following spots are clearly shown, and the group extended over about 10° of solar longitude. February 7 still further expansion was shown, with an extent of flocculus of about 15° longitude. Any other observations of this spot group on February 5 during the hours 10^h to 12^h 30^m would be welcome as affording information of the details of development.