Measurements of the O + to H + transition height and ion temperatures in the lower topside ionosphere over Arecibo for equinox conditions during the 2008–2009 extreme solar minimum

We present incoherent scatter radar measurements of electron density, electron and ion temperatures, and ion composition made at Arecibo Observatory (18.35°N, 66.75°W), which is at a geomagnetic latitude of 30°N (or 46.7° dip latitude), during the recent extreme solar minimum of 2007–2009 and find agreement between our data and recent reports of corresponding satellite observations. Both the in situ spacecraft measurements and our ground‐based radar profiles exhibit unusually low electron densities and cold temperatures. These two factors result in an extraordinarily contracted ionosphere and thermosphere. This contraction in the ionosphere in turn causes the O + / H + transition height to descend; thus, the base of the low‐latitude plasmasphere, or protonosphere, is found at extraordinary low altitudes. We show that during the geomagnetically quiet period of October 2009, the transition height h t , where [ O + ] = [ H + ] + [ He + ], was observed at altitudes as low as 800–820 km during daytime and descended as low as 450 km during the night. At night, when T e  =  T i  =  T n , temperatures below 675 K were measured at 03:00 Atlantic Standard Time. These values are about 100 K lower than corresponding temperatures observed by the Arecibo incoherent scatter radar during the previous solar minimum period (1995–1997).