HR: 1330h
AN: GP13B-08    [Abstracts]
TI: Evidence From Reversals and Excursions for a Role of Precession in the Geodynamo.
AU: * Fuller, M
EM: mfuller@soest.hawaii.edu
AF: HIGP-SOEST, U Hawaii,, 1680 East West Rd.,, Honolulu, HI 96822 United States
AU: Vanyo, J
EM: vnyo@engineering.ucsb.edu
AF: Dept. Geological Sciences., UCSB,, Santa Barbara, CA 93106 United States
AU: Dunn, B
EM: rdunn@geology.ucsb.edu
AF: Dept. Geological Sciences., UCSB,, Santa Barbara, CA 93106 United States
AB: Three aspects of geomagnetic field behavior suggest that the geodynamo is powered at least in part by lunisolar precession. First, excursions, or events, with field intensity lows during the last 500 kyrs, correlate with minima in the obliquity signal. Second, in the past 5 Myrs reversals occur preferentially when the amplitude of the obliquity signal is low. Third, the occurrence of the last reversal is located near to the minimum of the obliquity cycle in which it occurs. Despite earlier rejection of precession powered dynamos, discussions of dynamos powered by lunisolar precession of the mantle and core have been revitalized by the demonstration that (1) the available energy is more than sufficient and (2) the nature of the induced flows are likely to give field regeneration. The first order flow shows the core rotating as a nearly rigid sphere lagging the mantle in precession by a small angle. This gives a westward drift with respect to the mantle, consistent with the westward drift observed in secular variation. The lag generates relative velocities and displacements over the core mantle boundary as second order flows, which can through the coalescence of small vortices give patterns similar to flow patterns calculated near to the core mantle boundary. A third order flow of nested cylinders concentric to the core axis was also seen. Alternate cylinders move slowly prograde, or retrograde and alternate northward and southward internal flow. These geometries generated by precession are reminiscent of convective cells in a rotating liquid core because like them, they are dominated by the Coriolis force. The observation that the third order flow in the nested cylinders can stop and restart with the same, or a reverse sense, suggests a possible field reversal mechanism. Since transitional VGPs have preferred paths that appear to be related to regions with colder mantle above the core and these same regions are sources of persistent main field features, there must also be some role for convection. Thus the dynamo is probably driven by precessional flows that are modulated by convection and Coriolis dominated.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1510 Dynamo theories
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly