Earth's Main Field Research Articles

The reduction of magnetic observations to mean of year

Measurements of the Earth's main field are always affected by continuously varying electric currents flowing above and below the Earth's surface. During magnetic storms the Earth's field in low latitudes may be reduced by several per cent, and the gradual recovery of the field may continue for several days. The increase of field following the recovery‐period of a storm may persist for a week or more, so that the field may attain values considerably in excess of the annual mean value. This effect is often also found in the case of small and moderate disturbances of the Earth's field, and has caused serious difficulty in the task of estimating the changes in the main field appearing from year to year.

Asymmetrical characteristics of the Earth's magnetic disturbance‐field

Asymmetries in the Earth's magnetic disturbance‐field are considered in relation to the asymmetries in the Earth's main field, using data of 32 stations. The auroral‐zone curve of terrestrial magnetism, in north polar regions, is elongated in a direction roughly parallel to the elongation of Hsoclinic curves. It agrees roughly with the curve of maximum auroral frequency, as given by Fritz, except in regions where his auroral data were scanty. The curve appears to undergo regionally a small diurnal oscillation.The amplitude of the disturbance diurnal variation (SD) is approximately symmetrical with respect to the geographical equator in low latitudes, is zonal‐symmetrical with respect to the auroral zone, and has a zero vertical component near the north pole given by the eccentric dipole. No change in amplitude with longitude has been detected, and it is concluded that induced earth‐currents in the oceans contribute little to SD.The local time‐phase of the mainly sinusoidal variation SD shows a range of about four hours with longitude. But if in defining time the geographic north pole be replaced by the north pole given by the eccentric dipole, then at the auroral zone there is no significant variation in time‐phase with longitude. This suggests that the auroral zone has a controlling and initiating influence over the world‐wide electric currents responsible for SD.Average asymmetries in the geographical distribution of magnetic disturbance and aurora are compared. The average force‐vectors for the daily means of disturbance are perpendicular to the average directions of homogeneous auroral arcs, to a high degree of approximation, in polar regions.The SD electric current‐system of magnetic storms proposed by Chapman shows good general agreement with the extensive data here considered. Certain changes are suggested whereby the fit with observation may be improved.