The expansion of coronal plumes in the fast solar wind

Coronal plumes are believed to be essentially magnetic features: they are r ooted in magnetic flux concentrations at the photosphere and are observed t o extend nearly radially above coronal holes out to at least 15 solar radii , probably tracing the open field lines. The formation of plumes itself see ms to be due to the presence of reconnecting magnetic field lines and this is probably the cause of the observed extremely low values of the Ne/Mg abu ndance ratio. In the inner corona, where the magnetic force is dominant, steady MI-ID mod els of coronal plumes deal essentially with quasi-potential magnetic fields but further out, where the gas pressure starts to be important, total pres sure balance across the boundary of these dense structures must be consider ed. In this paper, the expansion of plumes into the fast polar wind is studied by using a thin flux tube model with two interacting components, plume and interplume. Preliminary results are compared with both remote sensing and s olar wind in situ observations and the possible connection between coronal plumes with pressure-balance structures (PBS) and microstreams is discussed .