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Session 2 - Everything Else.
Display session, Friday, June 27
Ballroom C, Chair: Richard Canfield

[2.13] Joint Instability of Latitudinal Differential Rotation and Concentrated Toroidal Fields below the Solar Convection Zone

M. Dikpati, P. A. Gilman (HAO/NCAR)

It was found by Gilman and Fox (July 20 1997 issue of ApJ) that the solar differential rotation below the convection zone is unstable to horizontal disturbances in the presence of a broad, nonuniform toroidal magnetic field. This instability occurs for longitudinal wave number 1. The primary energy source for the instability is the differential rotation for low field strength, and the toroidal field for high field strength. Since solar toroidal magnetic fields are inferred from sunspots to occur in narrow latitude belts, we have now studied the joint instability of solar differential rotation and highly concentrated toroidal field. The instability still occurs, and it occurs for both the longitudinal wave number 1 and higher wave numbers. The growth rates of the instability for some wave numbers greater than 1 are higher, if the toroidal field is in a belt equatorward of 60^\circ latitude. The primary source of energy for the instability is usually the magnetic field, but in some cases extraction of energy from the differential rotation becomes more important particularly for higher wave numbers with weaker fields. The instability occurs with high growth rate even for weak toroidal field when the field is at 30^\circ latitude or higher. This enhanced instability at high latitude may inhibit the build up of toroidal field preventing occurrence of sunspots at higher latitudes. For low latitudes, the much lower growth rate of the instability may allow sufficient toroidal field buildup that sunspots can be formed. The occurrence of this instability for wave numbers m=1 to 5 may help determine the observed distribution of active regions with longitude.

Program listing for Friday