It’s the magnetic field that extends from the earth’s interior to where it meets the solaria stream of charged particles emanating from the sun. The magnetic field resembles the field of a large bar magnet near its center or that due to a uniformly magnetized sphere. Its origin is thought to be generated deep down in the earth’s core.
At the surface of the earth, the pole of this equivalent bar magnet, nearest the north geographical pole is actually a south” magnetic pole. This paradoxical situation exists since by convention a north seeking end of a compass needle is defined as pointing north yet must point to a pole of opposite sense or South Pole of the earth’s magnetic field. The flux lines of the field exhibit the usually pattern common to a small magnet as shown below in fig 1.
The direction of the field lines at the equator is horizontal while at the north and south magnetic poles it’s vertical. This geometry is important in interpretation of magnetic anomalies. The earth total field intensity is not perfectly asymmetric about the geographical North Pole e.g. the north magnetic pole in northern Canada is more than 1000miles from the geographical pole.
The earth cannot be exactly represented by a single bar magnet but has numerous higher order poles and very large –scale anomalous features owing to unknown characteristics of the generating mechanism in the earth’s core. As shown in fig 1 the solar wind or constant flux of particles and electric currents from the sun distort the field lines. The deviation from asymmetric field is the anomalous set of features in the earth’s crust caused by local variations in the magnetic minerals and other features which distort the local earth’s magnetic field.
The magnetic field of the earth surface can be divided into three components i.e.:
1) Main field. It forms the largest component of the magnetic field and it’s caused by electrical currents in the earth’s fluid outer core. It acts as the inducing magnetic field in explorations.
2) External magnetic field. Forms a small portion of the observed magnetic field that is generated from magnetic sources external to the earth. Its produced by interactions of the earth’s ionosphere with the solar wind .As a result some temporal variations associated with the external magnetic field are correlated to solar activity.
3) Crust field. This forms the portion of the magnetic field that is associated with the magnetism of crustal rocks. It’s comprised of both magnetism caused by induction from the earth’s main magnetic field and forms remanent magnetization.
The earth’s magnetic field varies with time as illustrated in the working of the proton precession magnetometer. Significant time variations with periods of seconds, minutes and hours are the direct or indirect effect of the solar wind as it distorts the magnetosphere or external magnetic of the earth. Diurnal variations are not predictable; they occur over the course of the day and are...