SUBSTORM ELECTRON INJECTIONS - GEOSYNCHRONOUS OBSERVATIONS AND TEST PARTICLE SIMULATIONS

We investigate electron acceleration and the flux increases associated with energetic electron injections on the basis of geosynchronous obs ervations and test-electron orbits in the dynamic fields of a three-di mensional MHD simulation of neutral line formation and dipolarization in the magnetotail. This complements an earlier investigation of test protons [Birn et al., 1997b]. In the present paper we consider equator ial orbits only, using the gyrocenter drift approximation. It turns ou t that this approximation is valid for electrons prior to and during t he flux rises observed in the near tail region of the model at all ene rgies considered (similar to 100 eV to 1 MeV). The test particle model reproduces major observed characteristics: a fast flux rise, comparab le to that of the ions, and the existence of five categories of disper sionless events, typical for observations at different local times. Th ey consist of dispersionless injections of ions or electrons without a ccompanying injections of the other species, delayed electron injectio ns and delayed ion injections, and simultaneous two-species injections . As postulated from observations [Birn et al., 1997a], these categori es can be attributed to a dawn-dusk displacement of the ion and electr on injection boundaries in combination with an earthward motion or exp ansion. The simulated electron injection region extends farther toward dusk at lower energies (say, below 40 keV) than at higher energies. T his explains the existence of observed energetic ion injections that a re accompanied by electron flux increases at the lower energies but no t by an energetic electron injection at energies above 50 keV. The sim ulated distributions show that flux increases are limited in energy, a s observed. The reason for this limitation and for the differences bet ween the injection regions at different energies is the localization i n the dawn-dusk direction of the tail collapse and the associated cros s-tail electric field, in combination with a difference in the relativ e importance of E x B drift and gradient drifts at different energies. The results demonstrate that the collapsing field region earthward of the neutral line appears to be more significant than the neutral line itself for the acceleration of electrons, particularly for the initia l rise of the fluxes and the injection boundary. This is similar to th e result obtained for test ions [Birn et al., 1997b].