The theory of the nonlinear interaction between high- and low-speed solar wind streams has been limited either to small amplitudes or to the equatorial plane. It is shown how both of these restrictions can be removed for flow more than a few solar radii away from the sun by employing a quasi-radial approximation to develop a perturbation expansion of the equations of motion. The lowest-order equations describe the radial flow in large-amplitude latitude-dependent streams. Higher-order equations describe the nonradial flow associated with the streams and can be used to test the validity of the quasi-radial approximation for any given set of boundary conditions.

As an example, the theory is used to model a set of interplanetary scintillation observations indicating the disappearance, above 40¿N latitude, of the scintillation assoicated with a specific set of streams. The analysis illustrates the relative nonlinear steepening and displacement in phase of the density and velocity peaks as a function of latitude and time. Results are given in terms of the range of possible latitudinal variations at the sun and 1 AU allowed by the observations. It is concluded that a disappearance of the streams above 40¿N is not necessary to explain a weakening of stream interactions and thus the disappearance of scintillations at high solar latitudes.