Transient response of the solar wind to changes in flow geometry

The transient response of the solar wind to changes in geometry is examined. An initial stationary flow in a configuration that diverges as r ² is assumed. This state corresponds to the usual solar wind solution. The effect on the flow through a tube whose area A(r, t) diverges faster than r ², with the degree of divergence increasing in time, is considered. The asymptotic form of A(r, t) is chosen to mimic the form inferred in coronal holes. A detailed parameter study relating the form of A(r, t) to the pattern of flow in the tube is presented. It is observed that in the limit of large time (large compared to , the time constant for change in geometry of a flow tube) the solutions obtained from a time-dependent analysis can depend upon . For sufficiently large , the asymptotic solution is the same as the steady state solution obeying the correct boundary conditions and possessing a smooth sonic transition. However, if the geometry changes rapidly enough, solutions exhibiting shock-like discontinuities can also exist. This is essentially a new feature that emerges from the present investigation. Finally, it is suggested that this study may be useful in describing flows in evolving coronal holes.