DYNAMO THEORY AND PALEOMAGNETISM

For more than 200 years the origin of Earth's magnetic field was attri buted to permanent magnetization. Even today no single argument (e.g., that Earth's deep interior is too hot to sustain permanent magnetizat ion) conclusively rules out the permanent magnetization hypothesis. Ne vertheless, when all the evidence is considered, this hypothesis can b e safely discarded and replaced with an electric current (dynamo) hypo thesis. Surprisingly, this can be done even though there is no adequat e dynamo model for Earth. The development of geodynamo models began wi th the disk dynamo of Larmor in 1919 and expanded to include many clas ses of models, such as aw, alpha(2), alpha(2) omega, Taylor state, and Model Z dynamos. Because of mathematical difficulties associated with solving the many coupled partial differential equations of dynamo the ory, numerous simplifying assumptions are made. The majority of numeri cal dynamo models assume a three-dimensional velocity field in an invi scid fluid and use mean field theory to solve for axisymmetric magneti c fields. There is also an increasing number of intermediate and stron g field models emerging, in which feedback from the magnetic field to the velocity field is permitted. Nevertheless, these models still requ ire several simplifying assumptions and there are many additional prob lems. For instance, many core parameters are difficult to estimate; th ere is debate on whether the top of the core is stably stratified and on the effects such stratification might have; what effects the presen ce of an inner core have; and whether the coupling across the core-man tle boundary significantly affects the geodynamo. Perhaps it is not su rprising that dynamo theoreticians, faced with large difficulties in m athematics and many uncertainties in physics, essentially choose to ig nore input from fields such as paleomagnetism. However, it is precisel y because of such difficulties that paleomagnetism can provide valuabl e constraints to narrow the range of viable dynamo models. For example , paleomagnetism ultimately should provide constraints on the velocity and magnetic field symmetries of dynamos; determine whether the geody namo is in the weak, intermediate, or strong field regime; determine i f there is a fundamental difference in dynamo processes during superch rons when reversals of the magnetic field essentially cease; and provi de valuable information on the growth of the inner core and its possib le stabilizing effects on geodynamo processes.