Aspects of the current system producing magnetic disturbance

Abstract 1—If N, E. and Z (vertical) are the three rectangular components of the earth's magnetic field, a study of the behaviour of the changes ΔN, ΔE, and ΔZ produced in them by a perturbing field gives information about the corresponding changes in this field and therefore in the overhead current system to which the field is due. In this enquiry ΔN, ΔE, ΔZ will be regarded as departures for 60-minute intervals from an undisturbed condition which is taken to be the corresponding departure on quiet days. It will be understood that this procedure assumes that the effect of disturbance on the earth’s field is to overlay quiet conditions by an additional system of forces. Average diurnal variations on both quiet, q, and disturbed, d, days were therefore formed for N, E, and Z, starting from the published hourly values of the primary horizontal elements H and D where necessary. With the two sets of variations converted into departures from their respective means, difference departures Δd — Δq for N, E, and Z were formed hour-by-hour to give 24 disturbance field component vectors. This was done for the seasons separately and the year as a whole for each of the observatories in Table I. In discussing the average features of regular disturbance there are advantages in using as extensive series of data as possible. Even when the five internationally selected d and q days per month are used, as in the present enquiry, exceptional isolated disturbances may obscure normal behaviour. This is especially true when the study concerns stations representing transition zones in the phenomena of disturbance. The inclusion of Fort Rae, the British Polar Year station in N. W. Canada, will, however, be seen to be fully warranted, not because of the scale of disturbance there but because of its instructive position on the polar side of the line over which the current system producing the main features of the regular daily variation of disturbance is mainly concentrated.