We present the results of a controlled rocket‐borne experiment designed to study the very early time expansion characteristics (t < 1.0 s) of an artificially created ion cloud in the F region ionosphere. Using an attitude control system which optimized the investigation of parallel and perpendicular to B expansion properties, we have investigated the ionization and multi‐ion expansion features of the cloud constituents Ba+, Li+, and Ti+ and the relative influences of Saha and solar UV ionization mechanisms, ion‐neutral collisionality, and gyrokinetic orbits. Unique “in situ” diagnostic capabilities provided highly resolved measurements of the plasma structure within the cloud, the variations in mean ion composition, and the distributions of electron temperature. Among the findings we report that (1) the observations can be characterized by an expanding photoionized shell of structured cloud ions, with kinetically snowplowed background O+ ions and appreciable amounts of Li+ ions as forerunners on the leading and trailing edges; (2) a 25% depleted ionosphere is left behind the expanding shell; (3) the density structure within the cloud is created by gyrokinetic motion of individual ion components; and (4) electron temperatures are elevated on the average by 80% above the background levels with the hottest region (at twice the background temperature) in the domain of forerunner ions. The results are in agreement with simple analytical models and large‐scale simulations, corroborating and quantifying issues involving ionization, cycloidal dynamics, snowplow effects, depletion levels, and electron heating; while the unexpected discovery of Ti+ suggests an incomplete understanding of the burning process in the artificial cloud source and the temperatures in its ignition system.

Abstract. NASA sounding rocket 21.114, launched March 7, 1998, during the Coqui II campaign, provided neutral wind and plasma density measurements of a weak intermediate layer. The layer was centered near 140 km and had an approximate peak plasma density of 2200 cm -3. The measured winds were typically less than 40 rn s -t, in agreement with wind shear formation theory and coincident density observations. The data obtained during the flight allow us to explore the plasma density structure and wind field morphology of the intermediate layer. Coupled with simultaneous data from Arecibo Observatory, the upleg and downleg density profiles provide three spatially separated measurements that enable the first detailed investigation of the horizontal extent and variation of an intermediate layer.

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