It is shown that the existence of the so-called ''magnetic holes'' (magnetic field minima) along the field lines play a significant role in the plasma pressure balance in the Earth's magnetosphere. Ponderomotive forces induced by the Pc 1 geomagnetic pulsations propagating through these regions can lead to a significant plasma accumulation there. A numerical analysis of this effect based on an analytical model for the geomagnetic field is carried out. The variation of the redistributed plasma density versus radial distance, amplitude, and ratio of wave to ion cyclotron frequencies is presented. It turns out that density enhancements occur off the equator at high latitudes and that they increase when the McIlwain parameter L increases. The specific features of the Pc 1 propagation along the lines of forces in the presence of ''magnetic holes'' are discussed. It is suggested that the existence of regions with decreasing magnetic fields at high-altitude field lines can lead to resonance absorption of ion cyclotron waves (the ''magnetic beach'' effect). ¿ American Geophysical Union 1996