SIMULATION OF DISPERSIONLESS INJECTIONS AND DRIFT ECHOES OF ENERGETICELECTRONS ASSOCIATED WITH SUBSTORMS

The term ''dispersionless injection'' refers to a class of events whic h show simultaneous enhancement (injection) of electrons and ions with different energies usually seen at or near geosynchronous orbit. We s how that dispersionless injections can be understood as a consequence of changes in the electric and magnetic fields by modeling an electron injection event observed early on January 10, 1997 by means of a test -particle simulation. The model background magnetic field is a basic d ipole field made asymmetrical by a compressed dayside and a weakened n ightside. The transient fields are modeled with only one component of the electric field which is westward and a consistent magnetic field. These fields are used to model the major features of a dipolarization process during a substorm onset. We follow the electrons using a relat ivistic guiding center code. Our simulation results, with an initial k appa electron energy flux spectrum, reproduce the observed electron in jection and subsequent drift echoes and show that the energization of injected electrons is mainly due to betatron acceleration of the preex isting electron population at larger radial distances in the magnetota il by transient fields.