HR: 14:15h
AN: U42A-03 INVITED     [PDF]
TI: Dipole Decay, Secular Variation and Reversals
AU: * Bloxham, J
EM: jeremy_bloxham@harvard.edu
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AB: Over the last 160 years, the axial dipole component of the Earth's magnetic field has decayed by almost 10%, a rate of decay roughly an order of magnitude faster than would result from free decay. This decay rate is also characteristic of that required for geomagnetic reversal. Does the present-day dipole decay hold clues as to the mechanism of geomagnetic reversal? To address this question we begin with some simple kinematic and dynamical considerations of possible reversal mechanisms. Two end-member models of the reversal process can be eliminated, namely a purely diffusive reversal in which the dipole is destroyed through free decay and a purely advective reversal in which flux is exchanged between hemispheres. Both end-members would take too long: the first because of the long free-decay time, and the second becuase largescale flow across the geographical equator is extremely weak, owing to tangential geostrophy. Instead, reverse flux must grow in situ. As first shown by Gubbins in 1987, the rapid decay of the dipole over the last 160 years is almost entirely due to changes in the field at the core-mantle boundary beneath the South Atlantic Ocean, a region that is characterized by a large reverse flux patch. How do such reverse flux patches form and grow? One possible mechanism is flux expulsion, a process that involves both advection and diffusion. In a numerical model of the geodynamo, we observe two instabilities leading to flux expulsion, one of which results in mid-latitude reverse flux patches and is associated with reversal in the numerical model. Although this provides a tantalising connection between present-day secular variation and reversals there is no reason to believe that the present-day dipole decay will result in a reversal. We outline a method by which it might prove possible to predict the future behavior of the field.
DE: 1510 Dynamo theories
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
DE: 1545 Spatial variations (all harmonics and anomalies)
DE: 1560 Time variations--secular and long term
SC: U
MN: 2003 Fall Meeting