Adrian Hitchman

The quiet daily variation in the total magnetic field: Global curves

Most magnetic mapping exercises involve measuring the amplitude, or total-field component, of Earths magnetic field. Removing the time-varying part of the field is a task of data reduction, and the most common time variation is the quiet daily variation, Sq. It is thus valuable to have for reference type curves of the quiet daily variation in the total field. To meet this need, global data obtained during the 1965 International Year of the Quiet Sun have been used to derive type curves describing the Sq variation of the total magnetic field, in addition to the traditional field components. As for the traditional components, the total-field curves show significant seasonal and latitudinal variability in amplitude and phase. The effect of the equatorial electro-jet is clearly evident. In both hemispheres of the globe there are bands of reduced amplitude in total-field signal between the equator and the path of the Sq focus. These bands, here termed the total-field “doldrums”, persist throughout the year.

Time‐varying effects in magnetic mapping: Amphidromes, doldrums, and induction hazard

A magnetic amphidrome is defined as a place where changes of the magnetic field over time, as measured by a total‐field magnetometer, are reduced to the point of being negligible. The reduction is caused by destructive interference between the vertical and horizontal components of the time‐varying field. At an ideal amphidrome, variations with time are suppressed completely and the total‐magnetic‐field magnitude is steady. Such a phenomenon may be expected to depend on the frequency content of the time variations in the vertical and horizontal components. The subject is treated first in terms of the quiet daily variation, Sq, which is studied on a global basis. It is seen that there are magnetic latitude bands, north and south of the equator, where the quiet daily variation is minimal. These zones are called the “diurnal doldrums.” In addition to this global pattern, the magnetic daily variation is modified by Earth’s conductivity structure locally, and Sq amphidrome behavior may be aided or obstructed lo...

Broad-scale lithospheric structures of the Australian continent from 3- D inversion of observatory and magnetometer array

An exploratory 3-D model of the electrical conductivity structure of the Australian continent is presented. The model is derived from the inversion of vertical magnetic-field transfer functions from the Australia-wide Array of Geomagnetic Stations. The model reveals conductivity differences beneath Archaean cratons in Western Australia, enhanced-conductivity anomalies between Archaean cratonic regions and beneath Phanerozoic terranes in eastern Australia.