Improvements to the way data are taken and analyzed at the Incoherent Scatter Radar Facility at Arecibo, Puerto Rico, mean that the transition altitude, where the O+ and H+ ion densities are equal, can now be studied at all local times throughout the solar cycle. These data show that the diurnal variation of the O+-H+ transition height is characterized by a rapid collapse following sunset, which is associated with the contraction of the plasma due to the rapidly decreasing plasma temperatures at this time. A corresponding increase in the transition altitude is seen at sunrise. The topside data also reveal that a secondary collapse, which is a nonthermal effect, is often observed at local times near midnight. Measurements taken during October 1994 and 1992 are presented and compared to results from the Sheffield University Plasmasphere Ionosphere Model (SUPIM) to study the effect of increasing 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 transition 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 secondary collapse is less pronounced during solar minimum conditions. ¿ 1998 American Geophysical Union