7.4 Slanted contours
Although much of the debate in the early 1990s centered on discriminating between rotation constant on
cylinders and rotation constant along radial lines, neither picture gave a complete description of the data.
Gilman and Howe (2003) and Howe et al. (2005) pointed out that the differential rotation in the bulk of
the convection zone, at least at low- to mid-latitudes, could be quite well described by saying that the
contours of constant rotation lay at about a 25° angle to the rotation axis, as illustrated in
Figure 19: Mean rotation profile from GONG data; contours of constant rotation (left), showing
lines at 25° to the rotation axis as dashed lines, after Howe et al. (2005), and cuts at constant
latitude as a function of radius (right), after Howe et al. (2000b).
Figure 20 compares idealized rotation profiles for the cylindrical, radial, and slanted-contour
Figure 20: Idealized rotation profiles for rotation constant on cylinders (left), radial lines (middle),
and lines at 25° to the rotation axis (right). The top row shows contours of constant rotation,
while the lower row shows rotation rate as a function of radius at constant latitude for latitudes at
15° intervals from the equator (top) to 75° (bottom). The rotation rate is matched to the GONG
inferences at and smoothed to simulate the broadening effect of inversion resolution on
the tachocline; the near-surface shear was not included.