3.6 The base of the convection zone
Inversions to determine the radial profile of sound speed and other
structure quantities have been used to great effect in improving our understanding of the physics which goes
into solar structure models (e.g., Gough, 1996
; Christensen-Dalsgaard, 2002). In the context of solar
interior dynamics, the most important contribution of structure inversions has been to locate the base of the
solar convection zone at 
(Christensen-Dalsgaard et al., 1991), defined as the radius
at which the stratification changes from nearly adiabatic stratification to substantially sub-adiabatic
stratification (see Section 8.1). This result has until recently been viewed as very reliable but new elemental
abundance determinations have called it into question (Asplund et al., 2005; Bahcall et al., 2005).
Helioseismic estimates further suggest that the extent of the overshoot region below the convection zone is
no more than about 
of a pressure scale height, which is less than 
of the solar radius
(Monteiro et al., 1994; Basu, 1997). Basu and Antia (2001) find no significant variations in
either 
or the thickness of the overshoot region with latitude or time (variations in the
structure of the tachocline obtained from rotational inversions are discussed in Sections 3.2 and
3.3).
