A new approach is proposed to study the sensitivity of the Earth's magnetosphere to the variability of the solar wind bulk velocity. The study was carried out using a three-dimensional electromagnetic particle-in-cell code, with the microphysics interaction processes described by Maxwell and Lorentz equations, respectively, for the fields and particles. Starting with a solar wind with zero interplanetary magnetic field (IMF) impinging upon a magnetized Earth, the formation of the magnetospheric cavity and its elongation around the planet were modeled over time until a steady state structure of a magnetosphere was attained. The IMF was then added as a steady southward magnetic field. An impulsive disturbance was applied to the system by changing the bulk velocity of the solar wind to simulate a decrease in the solar wind dynamic pressure, followed by its recovery, for both zero and southward IMF. In response to an imposed drop in the solar wind drift velocity, a gap (air pocket) in the incoming solar wind plasma appeared moving toward Earth. The orientation of the cusps was highly affected by the depression of the solar wind for all orientation of IMF. The magnetotail lobes flared out with zero IMF due to the ‘‘air pocket'' effect. With the nonzero IMF, as soon as the gap hit the initial shock of the steady magnetosphere, a reconnection between the Earth's magnetic field and the IMF was noticed at the dayside magnetopause. 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 IMF was included. The expansion/ contraction of the magnetopause nose is almost linear in the absence of the IMF but evolves nonlinearly with a southward IMF. The system recovered its initial state on the dayside soon after the impulsive disturbance was beyond Earth for both cases of zero and nonzero IMF. Comparison with existing observations from Cluster and Interball-1 seems to confirm many of our simulation results.