We discuss here pitch angle distributions of energetic ions and electrons at substorm onset. The transition signatures of ion pitch angle distributions imply that energetic ions experience field-aligned acceleration after substorm onset which is directed either toward or away from the equator. Our studies indicate that the two often mentioned acceleration mechanisms-parallel electric field and convection surge acceleration-cannot explain the observed field-aligned acceleration signatures. An analysis of the parallel Lorentz equation of ions suggests that under the premise that there exist large temporal variations of the magnetic field, two magnetic field topologies are responsible for the observed field-aligned acceleration. ¿ 2000 American Geophysical Union