Aurora

Abstract

THERE was a fine aurora visible in this locality on Saturday night, April 23. It was seen at intervals, whenever the clouds broke away, until after midnight. This display is specially interesting, because it forms the continuation of a series of recurrences, at the precise interval of twenty-seven days, which began in December, the dates being as follows: December 9, January 5, February 2, February 29, March 27, and April 23. Some of these displays have been brilliant, and all of them have been well defined. In the table of auroras which I have constructed, based upon a periodicity corresponding to the time of a synodic revolution of the sun—namely, twenty-seven days, six hours, and forty minutes—there was, for several years preceding the sun spot minimum in 1889 and 1890, a return each spring of series of recurrences associated with the same part of the sun as that above described. A corresponding systematic tabulation of the records of solar conditions shows that this association bears a direct relation to reappearances at the eastern limb of an area which has been much frequented by spots and faculæ, and which has been located persistently south of the sun's equator. In like manner there are other areas located in the sun's northern hemisphere which have been much disturbed, and whose reappearances at the eastern limb have been attended year after year by series of recurrences of the aurora, in the autumn months chiefly, if not exclusively. From this it would appear that, in order that a solar disturbance may have its full magnetic effect upon the earth, it is necessary that it should be at the sun's eastern limb, and as nearly as possible in the plane of the earth's orbit. It appears, also, that the disturbances which recur upon certain parts of the sun so persistently year after year have greater magnetic effect than those of comparatively sporadic character located elsewhere.