THE GEOMETRIC SPREADING OF CORONAL PLUMES AND CORONAL HOLES

The geometric spreading in plumes and in the interplume region in coro nal holes is calculated, using analytic and numerical theoretical mode ls, between 1.0 and 5.0 R-.. We apply a two-scale approximation that p ermits the rapid local spreading at the base of plumes (fi) to be eval uated separately from the global spreading (f(g)) imposed by coronal h ole geometry. We show that fi can be computed from a potential-field m odel and f(g) can be computed from global magnetohydrodynamic simulati ons 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.