  The motion of a charged particle in a magnetic dipole has first been studied
by Stormer. The different applications of Stormer's theory to aurorae, cosmic
rays and Van Allen radiation belt particles are recalled in an historical
perspective. In this paper, we expand the Stormer theory in order to take into
account the effects produced by an additional uniform and stationary
interplanetary magnetic field (IMF) whose orientation is parallel or
antiparallel to the magnetic moment of the dipole. A new expression is derived
for the Stormer potential taking into account the additional IMF component. It
is shown how Stormer's allowed and forbidden zones are influenced by the
implementation of a northward or a southward IMF, and how a southward turning
of the IMF orientation makes it easier for Solar Energetic Particle and
Galactic Cosmic Rays to enter into the inner part of the geomagnetic field
along interconnected magnetic field lines.
