THE CRRES-AA-2 RELEASE - HF WAVE-PLASMA INTERACTIONS IN A DENSE BA+ CLOUD

An ionospheric chemical release, designated AA 2, was performed on Jul y 12, 1992, as part of the NASA Combined Release and Radiation Effects Satellite (CRRES) El Coqui rocket campaign. The purpose of the AA 2 e xperiment was to study the interaction between a powerful radio wave a nd a high ion mass (Ba+), ''collisionless'' plasma. Approximately 35 k g of Ba were explosively released near the center of the Arecibo high- frequency (HF) beam at 253 km altitude. This was the largest Ba releas e 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 KF beam produced the strongest Langmuir waves e ver detected with the Arecibo 430-MHz radar. Resonantly enhanced Langm uir waves were observed to be excited principally at the upshifted pla sma line (i.e., near 430 MHz + f(HF) where f(HF) is the frequency of t he modifying HF wave), and only weakly excited waves were apparent at the downshifted plasma line (430 MHz - f(HF). The upshifted plasma-lin e spectrum contained a dominant peak at the ''decay line,'' that is, a t the frequency 430 MHz + f(HF) - delta, where delta is close to the B a+ ion-acoustic frequency (similar to 2 kHz). Downshifted plasma-line echoes occurred at frequencies near 430 MHz - f(HF) and 430 MHz - f(HF ) - 1 kHz and exhibited little or no signal strength at the decay line (430 MHz - f(HF) + delta). During an initial period of intense upshif ted plasma-line excitation, the power asymmetry between the upshifted and downshifted plasma lines was of the order of 10(5) at the decay li ne. The upshifted plasma line was accompanied by strong HF-enhanced io n waves that were present only at the downshifted acoustic sideband. A fter geomagnetic field-aligned irregularities formed in the plasma the amplitudes of the upshifted and downshifted plasma lines equalized, a nd each exhibited spectra characteristic of the parametric decay insta bility. At early times in the Ba+ plasma the symmetry of wave excitati on anticipated for a parametric instability in a stationary, homogeneo us plasma was absent. The experimental results indicate that the devel opment of the parametric decay instability needs to be reexamined for a smooth plasma having a small (similar to 5 km) vertical scale length . Moreover, ion flow down geomagnetic field lines appears to suppress instabilities responsible for the formation of field-aligned irregular ities 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.