The existence of a layer of radial shear around the base of the convection zone, with approximately solid-body rotation below it, was first demonstrated by Brown et al. (1989), using the data of Brown and Morrow (1987); however, the significance of their results was quite low and they were at pains to point out that other interpretations of the data were possible. Dziembowski et al. (1989) used BBSO data to improve the picture of rotation at the base of the convection zone, again finding that the low-latitude rotation rate increased, and the high-latitude rate decreased, towards a common value at the base of the convection zone. The position of the base of the convection zone was determined by Christensen-Dalsgaard et al. (1991) using sound-speed inversions of helioseismic frequencies from the work of Duvall Jr et al. (1988) and Libbrecht and Kaufman (1988); their value of , confirmed by Basu and Antia (1997), has been accepted ever since.
The discovery of this shear layer (as pointed out by Brown et al.) offered a solution to the puzzle of the apparent absence of a radial gradient of rotation in the convection zone that could drive a solar dynamo, leading to speculation that the dynamo must operate in the tachocline region instead of in the bulk of the convection zone.
The tachocline lies in the region where modes of have their lower turning points, and the resolution of the inversions is quite low – about 5 – 10% of the solar radius in the radial direction. The thickness of the shear layer is therefore likely not to be resolved in inversions, and some ingenuity (and forward modeling) is required to estimate it and account for the effect of the finite-width averaging kernels in smoothing out the inversion inferences. The results of various efforts to parameterize the tachocline shape at the equator are summarized in Table 2. They mostly concur in placing the centroid of the shear layer slightly below the seismically-determined base of the convection zone, and its thickness at around . The largest value for the thickness, that of Wilson et al. (1996b), was obtained using forward calculation and direct combination of splitting coefficients rather than a true inversion, while the very low value of Corbard et al. (1999) was obtained using an inversion technique specifically designed to allow a discontinuous step in the rotation rate at the tachocline. The analysis of Elliott and Gough (1999) was somewhat different from the others, in that it involved calibrating a particular model of the tachocline against the inferred sound-speed rather than against a rotation profile.
|Wilson et al. (1996a)||0.68||0.01||0.12||–||BBSO|
|Antia et al. (1998)||0.6947||0.0035||0.033||0.0069||GONG|
|Corbard et al. (1998a)||0.695||0.005||0.05||0.03||LOWL|
|Corbard et al. (1999)||0.691||0.004||0.01||0.03||LOWL|
|Charbonneau et al. (1999)||0.693||0.002||0.039||0.002||LOWL|
|Elliott and Gough (1999)||0.697||0.002||0.019||0.001||MDI|
|Basu and Antia (2003)||0.6916||0.0019||0.0162||0.0032||MDI, GONG|
Antia et al. (1998) and Corbard et al. (1999) found no significant evidence for a variation in the position or thickness of the tachocline with latitude, but Charbonneau et al. (1999) found a significant prolateness, with the tachocline shallower at latitude 60° than at the equator. Basu and Antia (2003) also found a slightly thicker and shallower tachocline at high latitudes, and speculated that the tachocline location might be discontinuous at the latitude (around 30°) where the shear vanishes and changes sign.
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