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P. A. Gilman, M. Dikpati, M. S. Miesch (HAO/NCAR, 3450 Mitchell Lane, Boulder, CO 80301, USA)
Previous work has shown that in model 2D and 'shallow water' tachoclines containing latitudinally varying differential rotation and toroidal fields, a global MHD instability is present for tachocline type differential rotations and a wide range of toroidal field profiles (both broad and narrow in latitude), in which longitudinal wave number m=1 usually predominates, leading to a tipping of the toroidal field away from a purely E-W orientation. We show here that the same instability occurs in model tachoclines with similar differential rotation and toroidal fields, but for which the stratification is continuous in the vertical. The unstable disturbances here have periodic vertical structures up to wave numbers of at least 10, depending on the degree of 'subadiabaticity' of the stratification. For overshoot layer conditions, the modes of shallow water type dominate and may be the only ones present, requiring vertical displacement of the top boundary of the tachocline, while in the radiative tachocline, modes with a wide range of vertical wavenumbers grow, with the same growth rate in the asymptotic limit of strongly subadiabatic temperature gradient. The latitude and longitude variations of the unstable modes are identical to the corresponding structures of 2D or shallow water modes. They also have similar kinetic helicity profiles in latitude, but with vertical structure determined by the vertical wavenumber. These modes should also contribute to driving the solar dynamo. This work is partially supported by NASA grants W-10107 and W-10175. National Center for Atmospheric Research is sponsored by National Science Foundation.
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Bulletin of the American Astronomical Society, 36 #2
© YEAR. The American Astronomical Soceity.