The magnetic daily variation in Australia: dependence of the total-field signal on latitude

Abstract

The quiet daily magnetic variation, denoted Sq, occurs as a background signal during magnetic surveying. The morphology of Sq is dependent on a number of factors, particularly latitude. Type curves that describe the latitu-dinal dependence of the horizontal and vertical components of Sq on a global scale appear in many texts. This paper first describes a recent compilation of global Sq curves for the total magnetic field, which is the component of most relevance to magnetic mapping. An analysis is then made of total-field variations from a north-south line of stationary recording magnetometers which operated across central Australia as part of the AWAGS experiment of 1989-1990. These data are analysed for information on the magnetic daily variation across the Australian continent, particularly its latitudinal variation. Observations cover a full year and their analysis is divided into four seasons to show variation with season as well as with latitude. The observed data show a minimum in the total-field signal in the geomagnetic latitude band 20°?30°, in support of the global Sq total-field curves. There is also clear evidence in the AWAGS data for the path of the Sq focus across Australia, identified by an amplitude minimum in the daily variation of the horizontal magnetic north component. The band in which the total-field Sq variations are generally reduced is termed the total-field "doldrums"; this band is on the equator-side of the path of the Sq focus.