Interpretation of the electric fields measured in an ionospheric critical ionization velocity experiment

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Abstract
This paper deals with the quasi‐dc electric fields measured in the CRIT I ionospheric release experiment, which was launched from Wallops Island on May 13, 1986. The purpose of the experiment was to study the critical ionization velocity (CIV⋕ mechanism in the ionosphere. Two identical barium shaped charges were fired from distances of 1.99 km and 4.34 km towards a main payload, which made full three‐dimensional measurements of the electric field inside the streams. There was also a subpayload separated from the main payload by a couple of kilometers along the magnetic field. The relevance of earlier proposed mechanisms for electron heating in CIV is investigated in the light of the CRIT I results. It is concluded that both the “homogeneous” and the “ionizing front” models probably apply, but in different parts of the stream. It is also possible that electrons are directly accelerated by a magnetic‐field‐aligned component of the electric field; the quasi‐dc electric field observed within the streams had a large magnetic‐field‐aligned component, persisting on the time scale of the passage of the streams. The coupling between the ambient ionosphere and the ionized barium stream in CRIT I was more complicated than is usually assumed in CIV theories, with strong magnetic‐field‐aligned electric fields and probably current limitation as important processes. One interpretation of the quasi‐dc electric field data is that the internal electric fields of the streams were not greatly modified by magnetic‐field‐aligned currents, i.e., a state was established where the transverse currents were to a first approximation divergence‐free. It is argued that this interpretation can explain both a reversal of the strong explosion‐directed electric field in burst 1 and the absence of such a reversal in burst 2.