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

Abstract

The expansion of a coronal hole filled with a discrete number of higher-density 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 Alfvén waves propagating both inside and outside the structures. Remote sensing and solar wind in situ observations are used to constrain the parameter range of our study. Close to the Sun, the precise plasma parameters are fundamental in determining the relative position of the critical points, which are found by means of an iterative procedure because the flows from the two regions are coupled. At greater distances the filling factor of the higher-density regions may vary largely, and streams which are either faster or slower than the wind arising from the ambient hole may result, depending on the temperature differences and on the flux of Alfvén waves assumed in the two regions. Velocity differences of the order of ∼ 50 km s-1, such as those found in microstreams in the high-speed solar wind, might be thus easily explained by reasonable fluctuation amplitudes at the Sun, although the natural candidates for plumes at large heliocentric distances are more likely to be the so-called pressure-balanced structures. Copyright 1999 by the American Geophysical Union.