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SUMMARY:Magnetic Rossby waves in the Earth's core - Kumiko Hori (Leeds)
DTSTART:20150427T150000Z
DTEND:20150427T160000Z
UID:TALK58272@talks.cam.ac.uk
CONTACT:Jean Teyssandier
DESCRIPTION:Magnetohydrodynamic waves in rapidly rotating planetary cores 
 can produce several secular variations of the planetary magnetic field. So
 me axisymmetric modes\, including the torsional Alfven waves\, are thought
  to be responsible for certain observed features of the Earth’s core dyn
 amics and the geomagnetic variation. It is\, on the other hand\, possible 
 for other waves to lead to the nonaxisymmetric variations. A potential can
 didate is the magnetic Rossby wave\, which migrates in the azimuthal direc
 tion along the internal toroidal field. This can be related to the westwar
 d drift of the geomagnetic field\, which has been observed in the Atlantic
  hemisphere for the past hundred years. Though the drift has commonly been
  believed to reflect advection due to large-scale lateral flows beneath th
 e top of the core\, propagation of the waves excited within the core may a
 lso account for it. \n To investigate whether this mode can be relevant in
  the Earth’s core\, we extend a linear theory to quasi-geostrophic cylin
 ders and explore nonlinear dynamo simulations in rotating spherical shells
 . By performing tempo-spatial spectral analyses\, we identify the slow Ros
 sby waves that propagate at the correct speed\, given by the Alfven and Ro
 ssby speeds\, with respect to the mean zonal flow. The result indicates th
 at this mode could be excited in the planetary fluid core and that the wav
 e propagation may indeed play a role in the magnetic drift. This could giv
 e a framework for inferring the physical properties in the fluid core\, in
  terms of nonaxisymmetric waves.
LOCATION:MR14\,  Centre for Mathematical Sciences\, Wilberforce Road\, Cam
 bridge
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