Instability of Ekman-Hartmann boundary layers, with application to the fluid flow near the core-mantle boundary (vol 123, pg 15, 2000)

Our aim is to investigate the instability of mixed Ekman-Hartmann boundary layers arising in rotating incompressible magnetohydrodynamics flows in a p arameter regime relevant to the Earth liquid core. We perform a local study in a half space at a given co-latitude theta not equal pi /2, and assume a mean dipolar axial magnetic field with internal sources, Instabilities are driven, for high enough Reynolds number, by the quadratic term in the mome ntum equation. Following the work of Lilly [J, Atmos. Sci, 23 (1966) 481], we restrict our analysis to the linearized growth phase. We describe the de pendence of the critical Reynolds number in terms of theta and Elsasser num ber (measuring the relative strength of Lorentz and Coriolis terms). It is found that no matter how large the Elsasser number is, there exists a criti cal band centered on the equator in which instabilities can occur. For eeop hysically relevant values of parameters, this band extends over some 45 deg rees away from the equator. This study establishes the possibility of bound ary layer instabilities near the core-mantle boundary. (C) 2001 Elsevier Sc ience B.V, All rights reserved.