The Geometric Spreading of Coronal Plumes and Coronal Holes

The geometric spreading in plumes and in the interplume region in coronal holes is calculated, using analytic and numerical theoretical models, between 1.0 and 5.0 R. We apply a two-scale approximation that permits the rapid local spreading at the base of plumes (f_l) to be evaluated separately from the global spreading (f_g) imposed by coronal hole geometry. We show that f_l can be computed from a potential-field model and f_g can be computed from global magnetohydrodynamic simulations of coronal structure. The approximations are valid when the plasma beta is small with respect to unity and for a plume separation small with respect to a solar radius.