Effect of solar wind flow geometry on solar wind parameters at 1 AU for a low-frequency cascade driven solar wind

A two-fluid, one-dimensional solar wind model is used to explore how the change in the expansion of a given flow tube with heliocentric distance influences the properties of the solar wind. Using a single driving mechanism, namely ion cyclotron resonance through a nonlinear Kolmogorov cascade process, for proton heating, the Alfvén wave amplitude, electron density, and temperature are kept fixed at the coronal base, while the parameters defining the form and extent of the expansion are varied. The results of this parameter study show that it is the variation of the expansion with heliocentric distance rather than the overall faster than radial expansion of the flow tube that plays a critical role in determining the asymptotic properties of the solar wind. These results can be understood in light of the conclusions derived originally by Leer and Holzer [1980] with a simple ad hoc heating function, namely that the mass flux and flow speed at 1 AU depend on whether most of the energy is deposited in the subsonic or supersonic region of the flow. With a more complex heating function, such as the one considered in this study, it is shown how the location of the critical point changes as a result of the interplay between the location of resonance interaction and the expansion of the flow tube, which subsequently affects the energy distribution within that flow tube. In particular, it is shown that for sharp expansion both enhanced and inhibited flows can occur depending on the heliocentric distance of the expansion region.