NAIC/AO Newsletter, November 97
The campaign began in June with Arecibo's first experiment in generating API (artificial periodic irregularities) (PI, F. T. Djuth). The HF is first used to generate (in the D and E regions) irregularities with a spatial period equal to its own wave length, and then it is used as a radar to scatter from these irregularities and monitor their decay time. API was not easy to generate, possibly because naturally occurring partially reflecting layers were scarce.
Four experiments observed the characteristics of Langmuir turbulence, especially in the early times after the turn on of the HF (PIs Sulzer, Cheung, Lee, and Isham). Previous similar observations have shown that the strongest Langmuir turbulence occurs (at least at a 35 cm scale size) very near the HF reflection height. With the new higher power, the strongest signals are sometimes observed more than a kilometer below the reflection height, below the so called matching height which normally defines the lowest altitude at which the Langmuir turbulence is seen. Simulations of strong Langmuir turbulence predicted the spectral characteristics of the previous early time observations, although alternative explanations are still being developed. It will be interesting to see how these new observations can be explained; SLT simulations have not thoroughly explored the region below the matching height, and so it is extremely interesting to see how the comparisons will turn out.
Finally two experiments looked at different phenomena, the effects of the HF on the generation of spread F (PI Lee), and the excitation of the two plasmon decay instability (PI Cheung).
The ionosphere this summer was smaller than expected, sometimes approaching winter time conditions. This sometimes caused the experiments expecting summer conditions to terminate early in the evening. However, with our enhanced power we were able to generate very large temperature enhancements and large density cavities even in the summer evening. These are normally late night winter experiments. We expect the coming winter campaign to be very successful at generating airglow and the cavities. This campaign will be limited to only those experiments requiring winter solar minimum conditions and will consist of only sixteen observing periods.