MHD models of coronal plumes

Coronal plumes are believed to be essentially magnetic features: they are rooted in magnetic flux concentrations at the photosphere and are observed to extend nearly radially above coronal holes out to at least 15 solar radii, probably tracing the open field lines. Two MHD models are presented here, corresponding to two different physical regimes. In the inner corona, where the magnetic force is dominant, a 2-D linearization method is proposed and the observed strong superradial expansion which occurs near the base is modeled by a quasi-potential field. The corrections to the field lines due to the unbalanced pressure (plumes are denser than the surrounding region) are calculated and these grow with the heliocentric distance together with the increasing plasma beta. A thin flux tube model is then examined and the further expansion into the interplanetary medium is calculated by taking into account the total pressure balance across the plume-interplume boundary. Comparisons with both remote sensing and solar wind in situ observations are made and the possible connection between coronal plumes with pressure-balance structure (PBS) and microstreams is discussed.