Recent rocket probe, barium cloud and radar measurements conducted during equatorial spread F conditions are interpreted in terms of a Rayleigh‐Taylor gravitational instability operating on the bottomside of the F peak. The persistent theoretical problems associated with strong radar echoes typically observed in patch‐like structures at high altitudes are explained in terms of regions of depleted plasma density which bouyantly rise against the gravitational field.

The barium releases in the magnetotail during the Active Magnetospheric Particle Tracer Explorers (AMPTE) operation were monitored by ground‐based imagers and by instruments on the Ion Release Module. After each release, the data show the formation of a structured diamagnetic cavity. The cavity grows until the dynamic pressure of the expanding ions balances the magnetic pressure on its surface. The magnetic field inside the cavity is zero. The barium ions collect on the surface of the cavity, producing a shell. Plasma irregularities form along magnetic field lines draped over the surface of the cavity. The scale size of the irregularities is nearly equal to the thickness of the shell. The evolution and structuring of the diamagnetic cavity are modeled using magnetohydrodynamics theory.

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The second magnetospheric tail Ba release of the AMPTE program on May 13, 1985, was observed by several field stations. A Fabry‐Perot imager was operated at Mt. Hamilton, California, to measure the line‐of‐sight velocity of the barium ions in the tail. Simultaneous imaging observations were made from there and from El Leoncito in Argentina. From the two‐station imaging data sets we have obtained cloud position by triangulation. The ion cloud bulk velocity was obtained from the position measurements and was intercompared with the Fabry‐Perot direct velocity measurements. The triangulated barium ion cloud appeared to be field aligned, and its triangulated direction was in excellent agreement with the Tsyganenko‐Usmanov magnetic field model. Following the initial expansion phase and the magnetic cavity formation, the barium cloud became magnetized by the ambient magnetospheric magnetic field. The bulk of the ion cloud was moving very slowly compared to the ambient ion velocity, which was measured by the nearby IRM satellite and which was of the order of several hundred kilometers per second. The slow motion of the barium ions was attributed to an “electrostatic cavity” formation at the boundary of the high‐density cloud, which excluded the ambient electric field by polarization. Several morphological changes of the ion cloud were observed during the following period, which resulted in the bifurcation of the cloud and the formation of a distinct S shape. Thus the cloud appeared to exclude the ambient convection electric fields, and at the same time it remained responsive to some time‐dependent field configuration changes. Thirty‐five minutes after cloud release, the cloud suddenly brightened and accelerated in the antisunward direction, tending to take up the local plasma velocity. This acceleration coincided with an increase in the ambient magnetic field and the plasma velocity. There was no clear evidence that the change in the ambient conditions was a direct cause of the observed cloud behavior.