The motion of a plasmoid (plasma-field entity) across an inhomogeneous magnetic field distribution of which the direction and strength change along the penetration trajectory has been studied. The bulk velocity decreases when the plasma element penetrates into a region of increasing magnetic field. The critical magnetic field intensity where a plasmoid is stopped or deflected is found to be the same critical field as that which has been observed in laboratory experiments for a non-rotating B-field distribution. The polarization electric field induced inside a moving plasma element has been determined for both low-ß and high-ß plasmoids. The momentum density vector of a plasmoid is deflected in the—B x VB and—B x (B. ∇)B directions as it penetrates into an inhomogeneous B-field distribution. This kinetic model has been applied to the impulsive penetration of solar wind plasma irregularities impinging on the earth's geomagnetic field with an excess momentum density. As a consequence of impulsive penetration, a plasma boundary layer is formed where the intruding plasmoids are deflected eastward. Magnetospheric plasma is dragged in the direction parallel to the flanks of the average magnetopause surface. Diamagnetic effects of these impulsively penetrating plasmoids into the magnetosphere are also briefly discussed.