The upper atmosphere during June 5-11, 1991

Millstone Hill ionospheric incoherent scatter radar data collected during J une 5-11, 1991, are analyzed to determine F region neutral and ion composit ion, density, and temperature. The 10.7-cm solar flux index was at the very high level of 250, and the magnetic activity index Kp ranged up to the hig hest possible level of 9 during this period, making it of particular intere st for understanding: upper atmosphere behavior for such a nearly limiting combination of conditions experienced by the thermosphere. We find that the Mass Spectrometer Incoherent Scatter 86 (MSIS-86) model overestimates the daytime exospheric temperature and the neutral O concentration for this hig h level of solar activity. These results differ substantially from storm pe riods analyzed previously. The ionospheric F region converts from largely a tomic ions to largely molecular ions during the periods of strongest magnet ic activity. We show that this condition leads to erroneous determination o f neutral atmosphere parameters, including temperature, density, and drift; velocity, if not accounted for in incoherent scatter data analysis.