The CRRES AA 2 Release: HF wave‐plasma interactions in a dense Ba+ cloud

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Abstract
An ionospheric chemical release, designated AA 2, was performed on July 12, 1992, as part of the NASA Combined Release and Radiation Effects Satellite (CRRES) El Coqui rocket campaign. The purpose of the AA 2 experiment was to study the interaction between a powerful radio wave and a high ion mass (Ba+), “collisionless” plasma. Approximately 35 kg of Ba were explosively released near the center of the Arecibo high‐frequency (HF) beam at 253 km altitude. This was the largest Ba release of the CRRES experiments; it yielded a distinctive ionospheric layer having a maximum plasma frequency of 11 MHz. At early times (< 1 min after the release) the HF beam produced the strongest Langmuir waves ever detected with the Arecibo 430‐MHz radar. Resonantly enhanced Langmuir waves were observed to be excited principally at the upshifted plasma line (i.e., near 430 MHz + ƒHF, where ƒHF is the frequency of the modifying HF wave), and only weakly excited waves were apparent at the downshifted plasma line (430 MHz − ƒHF). The upshifted plasma‐line spectrum contained a dominant peak at the “decay line,” that is, at the frequency 430 MHz + ƒHF − δ, where δ is close to the Ba+ ionacoustic frequency (∼2 kHz). Downshifted plasma‐line echoes occurred at frequencies near 430 MHz − ƒHF and 430 MHz − ƒHF − 1 kHz and exhibited little or no signal strength at the decay line (430 MHz − ƒHF + δ). During an initial period of intense upshifted plasma‐line excitation, the power asymmetry between the upshifted and downshifted plasma lines was of the order of 105 at the decay line. The upshifted plasma line was accompanied by strong HF‐enhanced ion waves that were present only at the downshifted acoustic sideband. After geomagnetic field‐aligned irregularities formed in the plasma the amplitudes of the upshifted and downshifted plasma lines equalized, and each exhibited spectra characteristic of the parametric decay instability. At early times in the Ba+ plasma the symmetry of wave excitation anticipated for a parametric instability in a stationary, homogeneous plasma was absent. The experimental results indicate that the development of the parametric decay instability needs to be reexamined for a smooth plasma having a small (∼5 km) vertical scale length. Moreover, ion flow down geomagnetic field lines appears to suppress instabilities responsible for the formation of field‐aligned irregularities and may also have an impact on the way parametric instabilities are excited. New theoretical approaches are needed to resolve many of the issues raised by this experiment.