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A turbulent, high magnetic Reynolds number experimental model of Earth's core
Author(s) -
Zimmerman Daniel S.,
Triana Santiago Andrés,
Nataf HenriClaude,
Lathrop Daniel P.
Publication year - 2014
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2013jb010733
Subject(s) - physics , dynamo theory , rossby number , angular momentum , dynamo , reynolds number , dipole , magnetohydrodynamics , magnetic field , turbulence , magnetic reynolds number , lorentz force , mechanics , classical mechanics , computational physics , quantum mechanics
We present new experimental results from the University of Maryland Three Meter Geodynamo experiment. We drive a fully turbulent flow in water and also in sodium at magnetic Reynolds number R m = Δ Ω ( r o − r i ) 2 / η , up to 715 (about half design maximum) in a spherical Couette apparatus geometrically similar to Earth's core. We have not yet observed a self‐generating dynamo, but we study MHD effects with an externally applied axisymmetric magnetic field. We survey a broad range of Rossby number −68 < R o = Δ Ω / Ω o < 65 in both purely hydrodynamic water experiments and sodium experiments with weak, nearly passive applied field. We characterize angular momentum transport and substantial generation of internal toroidal magnetic field (the Ω effect) as a function of R o and find a rich dependence of both angular momentum transport and Ω effect on R o . Internal azimuthal field generation peaks at R o = 6 with a gain as high as 9 with weak applied field. At this Rossby number, we also perform experiments with significant Lorentz forces by increasing the applied magnetic field. We observe a reduction of the Ω effect, a large increase in angular momentum transport, and the onset of new dynamical states. The state we reach at maximum applied field shows substantial magnetic field gain in the axial dipole moment, enhancing the applied dipole moment. This intermittent dipole enhancement must come from nonaxisymmetric flow and seems to be a geodynamo‐style feedback involving differential rotation and large‐scale drifting waves.