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

A magnetic amphidrome is defined as a place where changes of the magnetic f ield over time, as measured by a total-field magnetometer, are reduced to t he point of being negligible. The reduction is caused by destructive interf erence between the vertical and horizontal components of the time-varying f ield. At an ideal amphidrome, variations with time are suppressed completel y and the total-magnetic-held 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, S-q, 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. Thes e zones are called the "diurnal doldrums." In addition to this global patte rn, the magnetic daily variation is modified by Earth's conductivity struct ure locally, and S-q amphidrome behavior may be aided or obstructed locally . The second part of the paper treats the magnetic "rapid fluctuations." A si mple condition for an amphidrome is that the direction of Earth's main magn etic field be parallel to the normal of the "preferred plane" in which the small vector changes of rapid magnetic fluctuations tend to lie. Examples a re given of observed data for Australia, and a numerical model of Australia n electrical-conductivity structure is used to predict amphidromes regional ly. Formal treatment of the preferred-plane concept involves taking the out -of-phase (or quadrature) part of the induction phenomenon into account as well, and a parameter is proposed which may be contoured to show an amphidr ome minimum. The phenomena of amphidromes are fundamental for magnetic mapping procedure s. Near amphidromes, the fluctuating magnetic fields of Earth are suppresse d, and their capacity for introducing error into magnetic survey data is re duced correspondingly. The case of a "complete" or "ultimate" amphidrome, applying to both diurnal and rapid fluctuations, may be expected to be rare. None is known at prese nt. The reason is that the diurnal doldrums, favoring S-q amphidromes, occu r at low latitudes. Rapid-fluctuation amphidromes. however, are more likely to occur at mid- to high latitudes.