A coronal hole is a region of the solar corona characterised by diverging magnetic fields of single polarity and lower-than-average densities (and probably temperatures). It is now generally accepted that coronal holes are the source of high-speed streams in the solar wind (Munro and Withbroe 1972, Kopp and Holzer 1976, Steinolfson and Tandberg-Hanssen 1977, Munro and Jackson 1977; see Pneuman 1980 for references to most of the theoretical and observational papers on coronal holes since their recognition in 1968). We consider an infinitesimal field-aligned flow tube with cross-sectional area A(r) where r is the heliocentric radius (Figure 1), and we adopt the functional form

A(r)/A(r ) = (r/r0)8

where r is the coronal base radius, and 5 is a parameter which measures the divergence of the flow. If s = 2 then the flow is purely spherically symmetric, while if s > 2 the flow is more strongly divergent as is expected to be the case for a coronal hole. The cross-section of a typical coronal hole is shown schematically in Figure 2.