At first the field was thought to be constant in direction and strength, but during the past century it became clear that this is not the case. For instance, a study on Be-10 in Pacific sediments by Lao et al. (1992) suggests that between 16 and 24 thousand years ago (i.e. the most recent ice age) the geomagnetic field was less intense than it is today, though the authors do not discuss the possibility that the Sun was magnetically less active during that time. The field is now again weakening in strength and at the present rate it will vanish in about 1500 yr (Hoffman, 1988). This weakening could just be a part of normal fluctuations in the field strength, but it could also be the beginning of a reversal of the geomagnetic field. That the field has flipped states often in the past is for instance shown by magnetic stripes on the sea floor: the sea floor grows as new hot rock is extruded at mid-ocean ridges, and as this rock cools it encodes the instantaneous polarity of the field; see Fig. 9. The youngest reversal took place 730,000 yr ago.
The polarity of the terrestrial magnetic field over the past 170 million years, recorded in the rocks of ocean floors. The present polarity is black; white is reversed polarity. [After Hoffman (1988).]
The geomagnetic field is seated in the outer core which consists of liquid material. The outer core, a 2300 km thick layer, is situated about 3000 km below the Earth's surface, where the temperature is about 4000 K and the dipole field strength about 5 G. A relic field would decay due to resistivity in only 10 to 100 thousand yr (Moffatt, 1978). Some kind of self-sustaining dynamo is therefore needed. In the outer core - "a slowly churning mass of molten metal sandwiched between the mantle of the Earth and the solid inner core" (Hoffman, 1988) - convection takes place because of the temperature difference across it. This temperature difference is maintained by a heating from below by radioactive decay processes in the inner core and by gravitational settling due to a density stratification. Convection together with the Earth's rotation can generate an alpha-effect in the outer core. There is little differential rotation so that the Earth could be an alpha^2-dynamo. The alpha-effect in such a dynamo results in a stationary magnetic field (Steenbeck and Krause, 1969b). It is uncertain, however, whether the heating of the outer core is sufficient to drive an alpha^2-dynamo. If not, differential rotation may play a role so that the Earth could also be an alpha-Omega-dynamo. Such dynamos can, for specific combinations of parameters, feature non-oscillatory solutions (e.g. Moffatt, 1978, Ch. 9) and thus an existing field can be sustained.
Dynamo action is believed to take place in other planets with a liquid conducting core as well, provided the rotation is fast enough (Venus, for example, rotates very slowly and has no magnetic field). See e.g. Moffatt (1978) and Parker (1979) for some more details, and Soward (1983) for a review on some model computations.
The cause of the reversals of the Earth's magnetic field is unknown. Reversals may originate from fluctuations in the turbulent convection (e.g. Parker, 1969; Levy, 1972), since the reversals are randomly spread over Earth's history (cf. Fig. 9). Once the field has reversed the dynamo again maintains the field in the same direction, until the next reversal occurs. What actually happens during a reversal is not known. Typical rates of change in the direction of the geomagnetic dipole are slow (a few degrees per century), so that a reversal would take several thousand years, but higher rates of change have been reported recently (see Fuller, 1989). The paths that the magnetic poles follow during a reversal are not well established (e.g. Hoffman, 1988, 1991; Bogue, 1991; McFadden, 1992; Jackson, 1992), so that it is unknown whether the dipole field itself moves around, or whether the dipole field disappears altogether for a while to reappear in opposite direction. The major problem for finding conclusive evidence is the lack of data and the poor distribution of data sites over the Earth's surface.
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