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49heights up to 10 or more solar radii by various coronal instruments in the EUV and VL channels (figure 1). They are remarkably quiescent structures and sometimes stay unchanged for days. Plumes originate in photospheric unipolar magnetic flux concentrations (DeForest et al. 1997), and highlight open magnetic structures in the corona. Over the origin, plumes expand super-radially (with a half-cone angle of 458) in their lowest 20–30 Mm, and more slowly above that, with a linear super-radial expansion factor of about 3 at 4–5 solar radii and about 6 at 15 solar radii. There is some indication that pressure balance structures which are a common feature in high-latitude, fast solar wind near solar minimum are remnants of coronal polar plumes (Yamauchi et al. 2002). Plumes are believed to be filled with a low-b plasma and are regions of plasma density enhancements. Across plumes, the total pressure balance should be kept and consequently they are regions of Alfvén speed minima. Lower than 10 solar radii, outward plasma flows in plumes are slower than in inter-plume regions, the speed difference can be up to several 100 km sK1 (Suess 1998). For coronal wave studies, polar plumes are interesting for several reasons. First of all, coronal holes are the regions of the solar wind acceleration which is observed to happen at heights below five solar radii. The acceleration can be con fre *v RSTar polar plumes are observed as bright quasi-radial rays in coronal holesMagnetohydrodynamic waves in coronal polar plumes BY VALERY M. NAKARIAKOV