The propagation of whistler mode wave packets in a cold magnetospheric plasma is investigated in the quasi-resonant regime (near the oblique lower hybrid resonance) using the ray approximation. The analysis is essentially simplified by introducing a special curvilinear orthogonal coordinate system (resonant coordinate system (RCS)), which is naturally adapted to the description of the propagation of quasi-electrostatic waves in a cold plasma, when the group velocity and the wave vector tend to be orthogonal to each other in the deep quasi-electrostatic limit. It is shown that in an overdense plasma of the Earth plasmasphere, the parameters of the RCS are independent of the space density distribution of the background plasma. The general expressions for the Hamiltonian and whistler wave field intensity focusing law in a deep quasi-resonant regime are derived.¿ 1997 American Geophysical Union