A simple model of a coronal hole

The coronal hole model of Parker (1963) is used to establish the new result that as the divergence of the flow tube increases the fluid expansion velocity increases throughout the flow, with certain parameter restrictions. Expressions are derived for the parameters of an infinitesimal field-aligned flow tube with boundary conditions requiring that the solution be critical and that the gas pressure go to zero as the radius goes to infinity. Coronal outflow velocities are presented for various values of the coronal base temperature and the polytropic index. The results demonstrate that the terminal velocity of the coronal stream increases as the divergence parameter of the flow tube increases. As the coronal base temperature decreases, the model accommodates a much greater terminal fluid velocity relative to its value for purely spherically symmetric flow.