The spatial dispersion pattern of particle precipitation during an event of sustained moderate magnetic activity was studied using observations of energetic (>30 keV) and auroral (0.3--20 keV) particles made by two low-altitude NOAA 6 and NOAA 7 spacecraft. The polar cap boundary (PCB, boundary between closed and open flux tubes) was identified on the nightside using the north-south asymmetry of solar energetic particle fluxes over the polar caps which were observed at that time. The energy and species dependence of the latitude of the boundary of energetic particles in the vicinity of the PCB was found to be consistent with a simple schema, being described by (1) the accelerator which provides the >30-keV particle population operates at the outer boundary of the plasma sheet and (2) the particles that are transported across magnetic flux tubes equatorward (earthward) by the convection electric field during their flight over a distance of approximately 50 to 100 RE from the equatorial region down to the ionosphere. These findings are in tune with the concept of a far-tail transient reconnection process which gives rise to suprathermal particles.

The poleward boundary of the total energy flux of keV electrons (at a threshold of 0.01 erg cm-2 s-1 sr-1) was often encountered at significantly lower latitudes than the energetic proton boundary, however. The equatorward shift of the auroral electron boundary was as much as that expected for the 5- to 10-keV protons, which suggests that there is not enough dense cold plasma in the outer tail plasma sheet to neutralize the electric space charge arising from the different convection trajectories of the earthward moving auroral protons and electrons, so that it appears that the auroral particles of both signs are forced to move together. The apparent convective displacement of auroral electrons (across the auroral zone) can be as large as 2¿ to 3¿ in latitude, which could shift their poleward boundary some 200--300 km equatorward of the actural PCB. ¿American Geophysical Union 1992