Magnetic stresses during the solar wind barium release of 27 December 1984

We reexamine the magnetic field measurements from the Ba⁺ ion release, made as part of the international AMPTE space programme on 27 December 1984. Observations from the Earth showed that the initial motion of the ion cloude was southward (-Z_GSE). Examination of the magnetic field structure at the two spacecraft in the vicinity of the release indicates that there was a net southward force acting during the period immediately preceeding the motion. We show that this force was adequate to accelerate the cloud in the southward direction. We propose that the momentum acquired by the core of the cloud in this process is balanced by ions on the flanks (in Y_GSE) accelerating northward with the stress being communicated through the field.