Abstract. Space weather effects on the ground are produced by magnetic field changes associated with electric currents resulting from the solar wind/magnetosphere interaction. For most ground events the principal currents involved are the auroral electrojet systems. However, on August 4, 1972, there was a major magnetic disturbance and an outage of the L4 cable system in the mid-western US that was thought to be caused by currents on the magnetopause. New model calculations now show that the observed magnetic disturbance was too localised to have been caused by magnetopause currents. Contour plots of the disturbance are instead consistent with an ionospheric current as the source. Equivalent current plots derived from the observed magnetic field variations show that a rapid intensification of an eastward electrojet was responsi ble for the magnetic disturbance and the cable system outage.
Magnetic fields created by a dc-electrified railway are a nuisance to the operation of a geomagnetic observatory and also disturb other electromagnetic studies. Theoretical formulas that enable quantitative estimates of the magnetic effect of a dc railway including leakage currents in the ground are presented in this paper. They are illustrated by numerical examples. The validity of the theoretical model was verified by measurements carried out in the vicinity of a nearly north-south railway in Calgary, Canada. The earth structure in that area is approximately layered, which is an assumption included in the theoretical model. The agreement between the measured magnetic fields due to trains and the theoretical values is good. Numerical computations indicate that magnetic fields larger than the maximum allowable noise level (assumed to be about 10 pT) at today's magnetic observatories may extend to distances of tens of kilometres from a railway. We have prepared computer programs based on the theoretical formulas in the MatLab, Octave, FORTRAN and IDL languages, in which the locations (i.e. the latitudes and the longitudes) of the point of observation, of the feeding substations and of the trains, together with the feeding and leakage currents and the heights of the feeding lines, can be given as inputs.
Stringent selection criteria using Kp indices, local magnetic activity estimates, and visual intercomparisons, followed by a statistical screening procedure have enabled the production of preliminary scalar and vector component magnetic anomaly maps derived from Magsat data at northern high latitudes, in spite of frequent external field disturbances. Comparisons between the maps and with other magnetic maps and geological information confirm their general validity.
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