8 Dynamics of the Tachocline and Overshoot Region
Although the distinction is sometimes blurred, the tachocline and the overshoot region are empirically
two very different things. Whereas the tachocline is defined helioseismically from rotational inversions
(Section 3.2), the overshoot region is usually defined in terms of the mean stratification and must be
probed instead with structural inversions (Section 3.6). The tachocline encompasses the overshoot region
but appears to be wider; whereas the upper tachocline may extend substantially into the convective
envelope at high latitudes, the lower tachocline lies below the overshoot region at all latitudes (Section 3.2).
What the two have in common is that they are both thin - according to current estimates,
the tachocline extends roughly a few percent of the solar radius while the overshoot region
occupies less than one percent. Thus, local-area and thin-shell models are particularly useful here
(Section 5).
