This paper is concerned with the structure of hydromagnetic waves with respect to finite plasma pressure and the curvature of field lines in those magnetospheric regions where magnetospheric parameters (Alfv¿n velocity, the quantity &bgr;, and equilibrium current) have extrema (the region of plasmapause and partial ring current). It is shown that hydromagnetic waves in these regions they can have the structure of a standing wave not only in the geomagnetic field direction but also across magnetic shells (such a structure is called the MHD resonator). It is hypothesized that radially polarized Pc4 pulsations as observed in the magnetosphere represent resonator-enclosed waves in the region of partial ring current. ¿ 1998 American Geophysical Union