Convection-driven quadrupolar dynamos in rotating spherical shells

Changes in earth's dipole

The dipole moment of Earth's magnetic field has decreased by nearly 9% over the past 150 years and by about 30% over the past 2,000 years according to archeomagnetic measurements. Here, we explore the causes and the implications of this rapid change. Maps of the geomagnetic field on the core-mantle boundary derived from ground-based and satellite measurements reveal that most of the present episode of dipole moment decrease originates in the southern hemisphere. Weakening and equatorward advection of normal polarity magnetic field by the core flow, combined with proliferation and growth of regions where the magnetic polarity is reversed, are reducing the dipole moment on the core-mantle boundary. Growth of these reversed flux regions has occurred over the past century or longer and is associated with the expansion of the South Atlantic Anomaly, a low-intensity region in the geomagnetic field that presents a radiation hazard at satellite altitudes. We address the speculation that the present episode of dipole moment decrease is a precursor to the next geomagnetic polarity reversal. The paleomagnetic record contains a broad spectrum of dipole moment fluctuations with polarity reversals typically occurring during dipole moment lows. However, the dipole moment is stronger today than its long time average, indicating that polarity reversal is not likely unless the current episode of moment decrease continues for a thousand years or more.

Hemispherical dynamos generated by convection in rotating spherical shells

At Taylor numbers of the order 10(8) and Prandtl numbers of the order 1, dynamos generated by convection are found for which the magnetic field is essentially confined to either the northern or the southern hemisphere of a rotating spherical shell. The time dependence is typically chaotic, but latitudinal waves can be discerned that cause the magnetic field to change its polarity in a cyclical fashion. A possible relationship to a solar phenomenon is pointed out.