Theory and observation of energetic ions in the lobes of the geomagnetic tail

In a recent paper, Owen et al. (1990, Planet. Space Sci.38, 851) presented a model which attempts to explain the nature of the pitch angle distributions of energetic ions within the lobes of the distant geomagnetic tail. There has, however, been little discussion of the relevant observations in the literature, and the motivation for the above mentioned work was largely based on unpublished observations plus a few examples presented in the paper itself. In this paper we aim to rectify this situation. After briefly reviewing the model and its results, we present energetic ( E ⩾ 35 keV>) ion data from the ISEE-3 spacecraft obtained during early 1983, when the spacecraft made several traversals of the distant geomagnetic tail ( x_GSE = - 230 R _E). The data demonstrate that during quiet periods in which the spacecraft is continuously located in the tail lobes, the pitch angle distribution is observed to be highly anisotropic, being peaked closely perpendicular to the magnetic field direction, but with a small net flow in the antisunward direction, in agreement with the model results. Further predictions of the model, concerning the variation of the lobe energetic ion distributions with position in the lobes, are compared to observations made as the spacecraft performed a traversal of the lobe. Finally, since the model indicates that a more isotropic distribution should exist in the tail lobe during solar particle events, we present data from such a period for further comparison. In all the above cases, good agreement is demonstrated between the data and the expectations of the model.