Bimodality in the climatological topside electron and exospheric temperature distributions at Arecibo

We report on the local time, occurrence frequency, and solar activity varia tions of the measured electron temperature and the ratio of the electron to ion temperatures at 589 km, and the inferred exospheric neutral temperatur es. The World Day measurements at Arecibo, Puerto Rico, were made between 1 985 and 1995, spanning both solar maximum and minimum conditions during pre dominantly quiet geomagnetic conditions. Histograms of the electron and exo spheric temperatures reveal bimodal distributions. The electron temperature bimodality is primarily due to the sharp difference between daytime and ni ghttime temperatures whereas the exospheric temperature bimodality is prima rily due to the difference between high and low solar activity. The nightti me exosphere and the electrons at 589 km are hotter by up to 400 K during h igh solar activity than during low solar activity. The daytime electrons at 589 km are, however, colder after 0800 LT during high solar activity than during low solar activity. The daytime ratio of electron to ion temperature s is also lower at high solar activity but not until after 1000 LT on avera ge. Unlike the electron temperature at 589 km, the distribution of the elec tron to ion temperatures is unimodal.