Particle acceleration by inductive electric fields in the inner magnetosphere

We review some features of charged particle dynamics during substorm dipolarization of the magnetic field lines in the inner magnetosphere. In the parallel direction, particles traveling at low latitudes are subjected to prominent equatorward-oriented acceleration due to the rapid direction-changing E x B drift. This centrifugal effect leads to focusing of ions with small parallel speeds into the equatorial region, which in turn yields the development of substantial (a few kV) parallel potential drops and the production of magnetic field-aligned electron beams. In the perpendicular direction, when the gyroperiod is comparable to the field-variation time scale, particles can experience abrupt nonadiabatic energization. As an example, O+ ions originating from the ionosphere are accelerated up to hundreds of keV while being injected earthward, hence providing a seed population for the outer ring current. These results suggest that the electric field induced by relaxation of the magnetic field lines can play an active role in the dynamics of the inner magnetosphere during the main phase of geomagnetic storms. (C) 2002 Elsevier Science Ltd. All rights reserved.