This study investigates the connection between neutral composition, plasma density, and electron temperature at F region heights and mid-latitudes by using in situ measurements obtained by instruments aboard the Esro 4 satellite. It is shown that considerable day-to-day variations in the neutral composition are closely followed by similar changes in the ionospheric plasma density; this is especially obvious during disturbed conditions. Furthermore, the expected inverse relationship between the electron temperature and plasma density is clearly shown in the data. The following sequence of events is suggested to occur during a negative ionospheric storm: A decrease in the O/N2 ratio causes a decrease in ionization, which in turn produces an increase in the electron temperature.