Separation of Magnetic Variation Fields and Conductive Structures in the Western United States

Summary The variation field of a polar substorm was recorded by an array of 42 variometers in the western United States. This field has been separated by surface integral methods into parts of external and internal origins, at four times in the time domain and at four periods in the period domain. It is shown that the anomalies in the vertical and east‐west horizontal variation fields are of internal origin and that the external fields vary smoothly over the array. The separated fields in both domains show internal currents induced in north‐south striking conductive structures by the east‐west horizontal field. Phase differences between the normal and anomalous fields are about 30°, and indicate large, highly‐conducting structures in which self‐induction controls the currents. The in‐phase normalized anomalous fields at period 89 minutes have been approximated by two‐dimensional models made up of upheavals of semi‐circular section and a step in the surface of a perfectly‐conducting half‐space. A semicircular upheaval of radius 150 km from an unperturbed level at depth 360 km models the anomaly related to the Southern Rockies. A step of height 120km at the Wasatch Front, together with a semi‐cylinder of radius 100 km under the Wasatch fault belt, give a good approximation to the observed anomaly at the Wasatch Front. The actual structures may be somewhat shallower and smoother. The real conductivity is estimated at 2.10 −12 e.m.u., a value which would be expected at temperatures near 1500°C. Such temperatures are reasonable at the depths concerned.