Millstone Hill ionospheric incoherent scatter radar data collected during June 5--11, 1991, are analyzed to determine F region neutral and ion composition, 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 highest possible level of 9 during this period, making it of particular interest 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 high level of solar activity. These results differ substantially from storm periods analyzed previously. The ionospheric F region converts from largely atomic ions to largely molecular ions during the periods of strongest magnetic activity. We show that this condition leads to erroneous determination of neutral atmosphere parameters, including temperature, density, and drift velocity, if not accounted for in incoherent scatter data analysis. ¿ 2000 American Geophysical Union