The diurnal variation of magnetic disturbance in high latitudes

1―The effect of natural disturbance on the earth’s magnetic field at any one place is at least twofold: (i) to introduce a regular variation (Sd ) periodic within the day and additional to, as well as different in type from (except in a limited region round the magnetic axis pole), the variation associated with quiet days (Sq ); and (ii) to suppose on Sd irregular changes which may either be of the distinctive type peculiar to large storms especially in low latitudes and generally preceded by the particular type of perturbation known as a sudden commencement, or the changes in the field may be of the apparently nondescript class which comprises an unlimited variety of short-period irregular oscillations. Of these effects of disturbances Sd is definitely a local time phenomenon: the sudden commencement with subsequent depression in the horizontal component of the field as definitely follows universal time. For the irregular and unclassified oscillations in moderate and high latitudes a diurnal variation in their incidence has been shown to exist for a few isolated localities. But in the general view it is not known whether this aspect of disturbance is controlled by local or universal time. Nor is it known whether the form of the diurnal variation in disturbance (which variation we shall denote by D) varies in any systematic way with latitude.