WORLD DAY OBSERVATIONS AT ARECIBO - 1985 TO 1989

In the past 5 years, simultaneous incoherent scatter and optical observations have been obtained at Arecibo, Puerto Rico, during fourteen World Day campaigns with an average duration of 3 or more days. Here we present the period average values of important F layer parameters such as the peak electron density and the height of the layer peak. We have compared our electron density measurements with those given by the international reference ionosphere and generally find a reasonable agreement. The meridional neutral wind velocity is calculated for a height of 300 km from the incoherent scatter measurements. The period average values of the vertical gradient in the meridional wind velocity is derived. A maximum positive gradient in this quantity and the vertical gradient in the vertical ion velocity are found to occur near to 0600 AST in most of the months we have examined. We find that these vertical gradients are some three times larger for solar cycle minimum conditions than they are at solar cycle maximum. This is a new result that, as far as we are aware, has not been reported before. The period average values of the southward and eastward ion velocities are also derived from the measured radar line-of-sight velocities and show no obvious solar cycle dependence. We have used an ion energy balance method to infer atomic oxygen densities and exospheric temperatures at Arecibo. The temperatures show a fair agreement with the mass spectrometer/incoherent scatter model. The neutral densities also agree fairly well for all the years up to 1989 but then are consistently less than those predicted by the model.