DETECTION OF HE+ LAYERING IN THE TOPSIDE IONOSPHERE OVER ARECIBO DURING EQUINOX SOLAR MINIMUM CONDITIONS

We describe recent developments in and results from topside incoherent scatter radar (ISR) measurements at Arecibo, PR, emphasizing helium i on measurements. Recent improvements in the data taking modes and the data processing permit isolation of the concentrations of oxygen, heli um and hydrogen ions between the F region peak and 2000 lan with about 10 minute time resolution. The need for the three ion non-linear leas t squares fits is justified by use of the goodness of fit; a two ion f it is shown to be unacceptable. The new measurements are optimized for the rapid height variations in the solar minimum nighttime ionosphere by replacing the traditional 1 ms pulse with a 500 mu s pulse. We sho w results from one day of a five day experiment during the Spring of 1 994. For these equinox solar minimum conditions, the altitude distribu tion of the helium ions usually has a maximum near the O+ to H+ transi tion altitude (h(t)), forming a distinct layer, most noticeable during the night. The maximum helium ion concentrations tend to be quite low , 2 or 3 x 10(3) cm(-3), or 10-20% of the topside plasma at the peak o f the He+ layer. The transition altitude, h(t), varies from about 1200 km to 1400 km during the day to near 500 km at night, and the He+ lay er follows this altitude variation. Finally we show that the location of the layer near h(t) and its intensification during the night can be explained using ambipolar diffusion equations.