Filtering of short-term fluctuations of the dynamo field through the electrically conducting mantle

Monday, 15 December 2014: 4:15 PM
Dominique Jault and Nathanaël Schaeffer, ISTerre, CNRS and University of Grenoble, Grenoble, France
Any information on the lowermost mantle conductivity obtained from the study of magnetic signals emanating from the core would help to constrain scenarios concerning the formation of the core and of the deep mantle or about ongoing chemical reactions at the core-mantle boundary (CMB). In addition, it is of interest to understand how short-term fluctuations of the dynamo field are eliminated through the electrically conducting mantle before searching for high-frequency waves or attempting to assimilate magnetic field data in dynamic models of the core.
The classical approach has consisted in prescribing the magnetic field at the CMB.
Then, the respective parts played by spherically symmetric conducting layers in the transformation of the core signal can be determined as a function of the distance to the core surface. An immediate observation is that conductivity of mantle rocks just above the CMB, where the field is prescribed, has negligible influence.
In this classical framework, writing the transfer function Γ(ω) as a truncated power series in the pulsation ω severely overestimates the screening of high frequency signals by the mantle.
However, the appropriate approach to the problem consists in coupling magnetic field solutions in the core and in the mantle. We rely on dynamo simulations to determine anew the respective role of the different conducting layers of the solid mantle in the transformation of the dynamo field.