The pitch angle distribution transition of energetic particles at substormonset observed by GEOS-2

We discuss here pitch angle distributions of energetic ions and electrons a t substorm onset. The transition signatures of ion pitch angle distribution s imply that energetic ions experience field-aligned acceleration after sub storm onset which is directed either toward or away from the equator. Our s tudies indicate that the two often mentioned acceleration mechanisms - para llel electric field and "convection surge" acceleration - cannot explain th e observed field-aligned acceleration signatures. An analysis of the parall el Lorentz equation of ions suggests that under the premise that there exis t large temporal variations of the magnetic field, two magnetic field topol ogies are responsible for the observed field-aligned acceleration.