On the variability of the quiet-day diurnal magnetic variation at Eskdalemuir and Greenwich

§ 1. The solar diurnal magnetic variation on quiet days (conveniently denoted by Sq ) undergoes more or lessregular changes of two kinds: one, affecting both type and amplitude, in the course of each year—an annual variation; the other, affecting chiefly the amplitude, in the course of the sunspot cycle—this will be termed the solar-cyclic change. In addition, it undergoes irregular or fortuitous changes from day to day. These have scarcely been studied as yet, and there is no precise systematic information available concerning them. They are of considerable interest owing to their probable connection with day-to-day changes in the sun’s ultra-violet radiation. We therefore propose to examine them systematically, especially with reference to their similarity at different stations. In this paper we confine ourselves to the records for Eskdalemuir and Greenwich, the respective geographical co-ordinates being:— Latitudr. Longitude. ˚ ' ˚ ' Eskdalemuir . . . . . . . . . . . . 55 19 N 3 12W Greenwich . . . . . . . . . . . . . 51 28 N 0 0 The material used consists of the daily ranges, in all three magnetic elements, on 819 very quiet days. It is shown that the ranges suffice for the comparison of different days, since (§ 15) the irregular changes in Sq , at any particular season and solar epoch, affect almost solely the amplitude of the daily variation, while the type, that is, the form of the curves representing the diurnal inequalities, remains constant.