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			Seminar

	"A DEEP DYNAMO GENERATING MERCURY'S MAGNETIC FIELD"

	Dec 1 (Fri)

	Ulrich Christensen
	Max-Planck Institute for Solar System Research
	Katlenburg-Lindau, Germany   
	Email: christensen@mps.mpg.de

Mercury has an internal magnetic field that is probably dominated by
large-scale axisymmetric components.  A dynamo operating in the fluid
part of Mercury large iron core is the most probable cause, but the
low field intensity, 1% of Earth strength, poses a problem for dynamo
theory. With the common assumption that Coriolis and Lorentz forces
balance in planetary dynamos, a 10-30 times stronger field is
expected. A new model of a dynamo driven by thermo-compositional
convection associated with inner core solidification is presented. The
thermal gradient at the core-mantle boundary is subadiabatic, hence
the outer region of the liquid core is stably stratified. The dynamo
operates only in its deep parts. Numerical simulations show that here
a strong field is generated. Because of the planets slow rotation it
is dominated by small-scale components. They fluctuate more rapidly
with time than the weak large-scale components. The dynamo field
diffuses through the stable conducting region where rapidly varying
parts are strongly attenuated by the skin effect, whereas the slowly
varying dipole and quadrupole components pass to some degree. The
model explains the observed structure and strength of Mercury surface
magnetic field and makes predictions that are testable with flying and
planned space missions.

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