The effects of meridional neutral winds on the O+-H+ transition altitude over Arecibo

Improvements to the way data, are taken and analyzed at the Incoherent Scat ter Radar Facility at Arecibo, Puerto Rico, mean that the transition altitu de, where the O+ and H+ ion densities are equal, call now be studied at all local times throughout the solar cycle. These data show that the diurnal v ariation of the O+-H+ transition height is characterized by a rapid collaps e following sunset, which is associated with the contraction of the plasma due to the rapidly decreasing plasma temperatures at this time. A correspon ding increase in the transition altitude is seen at sunrise. The topside da ta also reveal that a secondary collapse, which is a nonthermal effect, is often observed at local times near midnight. Measurements taken during Octo ber 1991 and 1992 are presented and compared to results from the Sheffield University Plasmasphere Ionosphere Model (SUPIM) to study the effect of inc reasing solar EUV flux on the topside ionosphere. It is shown that SUPIM is able to model reasonably well the sunset and sunrise behavior of the trans ition altitude. In addition, the secondary collapse of the O+-H+ transition altitude is also studied. It is shown that this secondary collapse may be directly correlated to the well-known "midnight" collapse phenomenon of the F region over Arecibo. The magnitudes of the secondary collapse under low and moderate solar activity are compared, and it is shown that the secondar y collapse is less pronounced during solar minimum conditions.