A number of geomagnetic observations indicate that the orientation of the interplanetary magnetic field (IMF) plays a definite role in triggering several magnetospheric processes. In this theoretical study based on the superposition of a dipole geomagnetic field model and an uniform IMF, it is shown that when the northward component of the IMF decreases (dBz/dt < 0), the geomagnetic cut-off surface moves to larger equatorial distances, the altitude of the mirror points of charged particles trapped in the geomagnetic field increases, and the value of their equatorial pitch angle decreases. Furthermore, we show that the loss cone angle of trapped particles decreases when dBz/dt is negative. The southward component of the magnetic field generated by an enhancement of the ring current produces similar effects everywhere inside this ring current loop.