VUME Upper Mantle of the Earth

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Geodynamo





Geodynamo - the self-sustaining process responsible for maintaining the earth's magnetic field in which the kinetic energy of convective motion of the earth's liquid core is converted into magnetic energy.
Paleomagnetic measurements suggest that the Earth has possessed a magnetic field for at least 3.5 billion years, yet the planet is not a permanent magnet: the field would vanish in just 100,000 years if it weren't being regenerated from the core. This generator is the geodynamo.
Earth's core of liquid iron circulates in a spiral flow, powered by thermal convection and twisted by Carioles forces from its rotation. Huge electric currents arise from this rotation, and these produce the field. The field lines are neat at the Earth's surface, forming the poles we're familiar with. In the liquid core, turbulence pulls the field into a wild knot of spaghetti. Since this process operates without an external energy source, the geodynamo is said to be self-sustaining.






In the 1940s and 1950s, W. M. Elsasser and E. N. Parker first elucidated the so-called alpha-omega mechanism, by which core fluid motion can act as a dynamo if it consists of a combination of differential rotation and convective helical motion.
In 1995 Gary Glatzmaier (UCSC) and Paul Roberts (UCLA) created the first self-consistent time-dependent simulation of this process in 3D.  Their model works: it predicts the poles and the dynamic interior, as well as the occasional global polarity flip.  Movies are here.

Geomagnetic reversals





When a rock is formed it usually acquires a magnetisation parallel to the ambient magnetic field, i.e. the core-generated field. From careful analyses of directions and intensities of rock magnetisation from many sites around the world it has been established that the polarity of the axial dipole has changed many times in the past, with each polarity interval lasting several thousand years. These reversals occur slowly and irregularly, and for a period of about 30 million years around about 100 million years before present, there were no reversals at all. In addition to full reversals there have been many aborted reversals when the magnetic poles are observed
to move equatorwards for a while but then move back and align closely with the Earth's spin axis. The solid inner metal core is thought to play an important role in inhibiting reversals. At the present time we are seeing a 6% decline in the dipole moment per century. Whether this is a sign of an imminent reversal is difficult to say.
Westward drift.
Using direct observations of the magnetic field over the past 400 years, the pattern of declination seen at the Earth's surface appears to be moving slowly westwards. This is particularly apparent in the Atlantic hemisphere at mid- and equatorial latitudes. This may be related to the motion of fluid at the core surface slowly westwards, dragging with it the magnetic field lines.