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4 - Geomagnetic excursions
Published online by Cambridge University Press: 23 September 2009
Summary
Introduction
In addition to polarity changes, the Earth's magnetic field has often departed for brief periods from its usual near-axial configuration, without establishing, and perhaps not even instantaneously approaching, a reversed direction. This type of behaviour has been called a geomagnetic excursion. Geomagnetic excursions have been reported in lava flows of various ages in different parts of the world and from some deep-sea and lake sediments. Excursions are generally observed to commence with a sudden and often fairly smooth movement of the virtual geomagnetic pole (VGP) towards equatorial latitudes. The VGP may then return almost immediately, or it may cross the equator and move through latitudes in the opposite hemisphere before swinging back again to resume a near-axial position. Barbetti and McElhinny (1976) define the term ‘excursion’ to describe a VGP movement of more than 40° from the geographic pole (following the suggestion of Wilson et al. (1972) for intermediate pole positions) which terminates with a return of the Earth's field to its pre-existing polarity, without the dynamo being observed to establish itself in the opposite polarity. Defined in this way, excursions are differentiated from the secular variation (when the VGP co-latitude θ<40°) and from short polarity events – a term Barbetti and McElhinny apply only when the opposite polarity (θ<40° or θ>140°) persists long enough for at least one oscillation in the strength of the main dipole field (about 104a). It is possible that excursions represent abortive reversals.
It is not always easy to distinguish between an excursion and a short polarity event. Indeed, as we shall see later, it is a moot point whether there are any real differences in the physical processes that initiate reversals.
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- Reversals of the Earth's Magnetic Field , pp. 87 - 123Publisher: Cambridge University PressPrint publication year: 1994
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