Coronal plumes and the expansion of pressure-balanced structures in the fast solar wind

The expansion of a coronal hole filled with a discrete number of higher-den sity filaments (plumes) in overall pressure balance with the ambient medium is described within the thin flux tube approximation. The resulting solar wind model extends the results of Parker [1964] and Velli et al. [1994] to nonisothermal temperature profiles and includes a flux of Alfven waves prop agating both inside and outside the structures. Remote sensing and solar wi nd in situ observations are used to constrain the parameter range of our st udy. Close to the Sun, the precise plasma parameters are fundamental in det ermining the relative position of the critical points, which are found by m eans of an iterative procedure because the flows from the two regions are c oupled. At greater distances the filling factor of the higher-density regio ns may vary largely, and streams which are either faster or slower than the wind arising from the ambient hole may result, depending on the temperatur e differences and on the flux of Alfven waves assumed in the two regions. V elocity differences of the order of similar to 50 km s(-1) such as those fo und in microstreams in the high-speed solar wind, might be thus easily expl ained by reasonable fluctuation amplitudes at the Sun, although the natural candidates for plumes at large heliocentric distances are more likely to b e the so-called pressure-balanced structures.