In a magnetically closed magnetosphere, none of the geomagnetic field lines intersects the magnetopause. Solar wind particles incident upon such a field topology are normally thought to be specularly reflected and therefore none gains entry to the magnetosphere. Strictly speaking, however, specular reflection occurs only when the incident charged particle encounters a uniform magnetic field. Although the geomagnetic field at the magnetopause does not appear quite uniform to incident particles (its scale size is much greater than the gyroradii of wind particles), it possesses a small gradient parallel to the magnetopause. When this gradient is taken into account, a fraction of the incident particles can gain access to the magnetosphere. This gradient drift entry (GDE) process is a feature of the basic structure of the geomagnetic field and should occur at all times. It is therefore expected that the Active Magnetospheric Particle Tracer Explorers (AMPTE) lithium ion releases will penetrate certain regions of the magnetopause. An excellent test of the GDE process could be made by initiating a lithium release during a northward interplanetary magnetic field (when the magnetosphere is thought to be magnetically closed).