BARIUM CLOUD EVOLUTION AND STRIATION FORMATION IN THE MAGNETOSPHERIC RELEASE ON SEPTEMBER 21, 1971

摘要

The joint NASA-Max Planck Institute Barium Ion Cloud (BIC) Experiment on September 21, 1971, involved the release of 1.7 kg of neutral barium at an altitude of 31 500 km at a latitude of 6.93° N. and a longitude of 74.40° W. A theoretical model describing the barium neutral cloud expansion and the ion cloud formation is developed. The mechanism of formation of the striational features observed in the release is also discussed. Two candidate instabilities, which may contribute to striation formation, are examined. The drift instability stemming from the outwardly directed drag force exerted on the ions by the out streaming neutrals is rejected on the grounds that the ion density is too low during the collision-dominated phase of the cloud expansion to support this kind of instability. The joint action of Rayleigh-Taylor and flute instabilities plausibly accounts for the observed striational structure. This same mecha¬nism may well be operative at times of sudden injection of plasma into the inner magneto-sphere during geomagnetic storms and may thus contribute to the formation of field-alined inhomogeneities which serve as whistler ducts.