In a companion paper we have reported results from a survey of strong energetic ion (E>25 keV) anisotropies in the central geomagnetic tail based on 2 years of data from the ISEE 2 spacecraft. Here we extend this study with particular emphasis on the relation of high ion anisotropies to recent models of geomagnetic substorms and include magnetic field and ground-based data. Beyond 16 RE downtail distance, we find a correlation between the magnetic field north-south component (Bz) and the streaming direction of the energetic ions. Earthward ion streaming is observed almost exclusively when Bz is positive while tailward streaming tends to occur with negative Bz. Strong ion anisotropies are magnetic field aligned rather than perpendicular to it suggesting that most of them are observed at the plasma sheet boundary. However, they are also found in the central plasma sheet. A superposed epoch analysis using the AL index yields a relation between substorm phases and the observation of high ion anisotropies. Earthward streaming at larger distances from the Earth and/or the neutral sheet usually occurs during substorm recovery while earthward streaming closer to the Earth and tailward streaming events are related to the substorm expansion phase. Three events characterized by strong tailward ion and plasma streaming detected first changing to earthward streaming later on are presented in some detail.

The overall features of two of these events strongly suggest the formation of a magnetic neutral line earthward of the satellite at substorm onset moving tailward around recovery. While most observations during the third event also indicate such a sequence, there are some puzzling details which do not match this interpretation. Although a certain By signature proposed by numerical simulations of near-Earth reconnection could not be found, we conclude that the statistical properties of strong ion anisotropies can consistently be explained in terms of the near-Earth neutral line model of substorms if the substorm neutral line sometimes forms well inside the ISEE orbit. Other recently published models of substorm onset such as the thermal catastrophe model and the current disruption model can also account for part of the observations but still require further refinement. ¿ American Geophysical Union 1993