Eigen Ultra-Low-Frequency Magnetosonic Oscillations of the Near Plasma Sheet

A critical analysis of existing theories of the magnetospheric resonator for fast magnetosonic waves is performed. A new variant of the theory is suggested, according to which the near-Earth part of the plasma sheet plays the role of the resonator. It is shown that the magnetosonic wave is locked inside this region over its entire boundaries. The eigen frequencies of resonator modes are in a good agreement with observed values (0.8, 1.3, 1.9, 2.6, 3.4, ... MHz), both when estimated in the order of magnitude and when calculated exactly in the context of a simple model.