Any change in the earth’s magnetic field which is a function of time.

Related Terms


A determination of the magnetic field at the surface of the earth by means of ground-based instruments.


An anomaly of the magnetic field of the earth, extending over a relatively small area, due to local magnetic influences. Also called LOCAL ATTRACTION, MAGNETIC ANOMALY.


Phenomenon exhibited by some solids in which the deformation of the solid depends not only on the stress applied to the solid but also on the previous history of this stress; analogous to magnetic hysteresis, with magnetic field strength and magnetic induction replaced by stress and strain respectively.


Neutralization of the strength of the magnetic field of a vessel, using electric coils permanently installed in the vessel.


A temporary magnet made of a ferromagnetic core wound with insulated wire through which is passed an electric current. The current flow through the windings causes a magnetic field to build that will attract magnetic material.


The effect caused by the interaction of the magnetic field of the current carrying conductors in the armature with the main magnetic field. The two magnetic fields combined cause a distortion in the overall magnetic field that results in a shift in the neutral plane in the direction of armature rotation.


The amount of secular change in the earth’s magnetic field which occurs in 1 year. magnetic annual variation; the small systematic temporal variation in the earth’s magnetic field which occurs after the trend for secular change has been removed from the average monthly values.


A magnetic needle used in compass adjustment to find the relative intensity of the horizontal components of the earth’s magnetic field and the magnetic field at the compass location. Also called HORIZONTAL FORCE INSTRUMENT.


A proximity sensor which uses an alternating magnetic field to create eddy currents in nearby objects, and then the currents are used to detect the presence of the objects.


Omega signals propagate in the earth-ionosphere wave guide. This waveguide can support many different electro- magnetic field configurations, each of which can be regarded as an identifiable signal component or mode having the same signal frequency, but with slightly different phase velocity. Modal interference is a special form of signal interference wherein two or more waveguide modes interfere with each other and irregularities appear in the phase pattern. This type of interference occurs predominantly under nighttime conditions when most of the propagation path is not illuminated and the boundary conditions of the waveguide are unstable. It is most severe for signals originating at stations located close to the geomagnetic equator. During all daylight path conditions, the only region of modal interference is a more-less circular area of radius 500-1000 kilometers immediately surrounding a transmitting station.

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