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P. A. Gilman, M. Rempel, M. Dikpati (HAO/NCAR)
Recently Dikpati & Gilman (2001, ApJ, 552, 348) have shown, using a shallow-water model of the solar tachocline that allows the top surface to deform, that a tachocline with the observed broad differential rotation and a strong toroidal field is prolate. A strong toroidal field ring requires extra mass on its poleward side to provide a hydrostatic latitudinal pressure gradient to balance the poleward curvature stress. In a parallel study using a different approach, Rempel et al (2000, A&A, 363, 789) have shown that a weakly subadiabatic stratification leads to a complementary equilibrium state of the overshoot tachocline in which the magnetic curvature stress is balanced by a prograde rotational jet inside the toroidal ring. We show that the shallow water model yields a similar equilibrium state if we suppress the shell deformation and allow the differential rotation to be modified. We are analyzing the stability of such an equilibrium tachocline by using the MHD shallow-water model of Gilman & Dikpati (2002, ApJ, submitted). We expect to show that the combination of toroidal band and rotational jet is virtually always unstable to disturbances with longitudinal wave number m>0, except perhaps when the band is extremely narrow. This instability could wipe out the jet, and lead to some poleward migration of the toroidal field, as well as the excitation of longitudinally periodic magnetic patterns that might provide sites for magnetic bouyancy to produce spots as well as other photospheric magnetic features. This work is supported by NASA grants W-19752 and S-10145-X. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
The author(s) of this abstract have provided an email address for comments about the abstract: dikpati@hao.ucar.edu
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.