Abstract : new approach, using a 3D electromagnetic particle-in-cell (PIC) code, is presented to study the sensitivity of the Earth's magnetosphere to the variability of the solar wind bulk velocity. Starting with a solar wind impinging upon a magnetized Earth, time was let to the system so a steady state structure of the magnetosphere was attained. Then an impulsive disturbance was applied to the system by changing the bulk velocity of the solar wind to simulate a depression in the solar wind dynamic pressure, for zero, southward and northward interplanetary magnetic field (IMF). As a result of the applied disturbance, an air pocket effect that could be described as a ~15 Re wide gap is formed for all cases of IMF condition. As soon as the gap hit the initial bow shock of the steady magnetosphere, a reconnection between the Earth's magnetic field and the southward IMF was noticed at the dayside magnetopause (MP). During the expansion phase of the system, the outer boundary of the dayside magnetopause broke up in the absence of the IMF, yet it sustained its bullet shape when a southward and a northern IMF were included. The time relaxation of the MP for the three IMF cases was studied. The code is then applied to study the Halloween event of October 2003. Our simulation produced a new kind of air pocket, a rarefied space that was generated following a strong gradient in the impinging IMF. Such a feature is quite similar to observed hot flow anomalies and may have the same origin.