On the bulk motion of the ion clouds formed by the AMPTE solar wind/magnetosheath releases

Abstract

A principal objective of the Active Magnetospheric Particle Tracer Explorers mission was to release lithium and barium ion clouds that were initially sufficiently mass dense to strongly perturb the ambient solar wind or magnetosheath flow. A key property of these release clouds was that their spatial and temporal scales were smaller than, or of the order of, the Larmor scales of all the ion species involved. A one‐dimensional hybrid simulation study conducted by Chapman and Schwartz (1987) showed that momentum could be transferred locally from the oncoming protons to the majority of the release ions, which are associated with the diamagnetic cavity produced by the release, via the quasi‐steady boundary layer that forms at the cavity edge. The direction of motion of these “snowploughed” release ions and the field structure that accelerates them is just that of the oncoming protons. The remainder of the release cloud is photoionized outside of the diamagnetic cavity, and as a consequence of the “pickup” interaction of these ions in the oncoming proton flow the protons are deflected from their ambient flow direction in the release cloud vicinity. This deflected direction of the oncoming proton flow, just upstream of the snowplough fields, then defines the direction of motion of the snowploughed release ions, which eventually comprise the release ion cloud that is observed from the ground. This transverse component to the direction of the release ion cloud motion suggested by the above simple picture is qualitatively consistent with ground‐based observations of the barium releases. Here this simple model for the snowplough dynamics is discussed quantitatively for the particular conditions of the various lithium and barium releases. In particular, the model is found to accurately predict the time taken for the snowplough to return to the Ion Release Module (IRM) (the release spacecraft) location after the initial expansion of the cloud, an event which, as suggested from both the Chapman and Schwartz simulation results and the IRM in situ data, corresponds to the return of the magnetic field at the IRM. The magnitude of the transverse displacement of the December 27, 1984 barium release ion cloud that was seen from ground‐based optical observations is also predicted by results from the simple model. Finally, the model suggests that for the barium releases, ∼10‐15% of the total number of ions released were photoionized upstream of the snowplough field structure and were therefore available to balance the transverse momentum of the snowploughed release ion cloud, via the deflected proton flow. Simple considerations suggest that this fraction should be adequate to balance the observed transverse momentum of the release ion cloud.