The early effects of increasing solar activity upon the temperature and density of the 1000‐kilometer ionosphere

Observations of electron temperature and concentration at 1000 km carried out continuously since October 1964 by electrostatic probes on Explorer 22 have permitted certain ionospheric effects of increasing solar activity to be detected. Midday and midnight measurements throughout the latitude range of approximately 60°N to 60°S for one‐month periods in January 1965, 1966, and 1967 are presented. Comparison of these latitude profiles shows that the daytime electron concentration at 1000 km increases with solar activity at all latitudes, while the electron temperature increases initially (in the period from 1965 to 1966) but then decreases (1966 to 1967). The nighttime ionosphere is less affected by the increased solar activity, although N e decreases somewhat, and T e increases slightly. This behavior of the 1000‐km ionosphere can be understood qualitatively in terms of the changing composition and density of the neutral atmosphere, which in turn is caused by the increase in neutral temperature during this period of increased solar activity. A factor of prime importance for the electron temperature is the escape of neutral hydrogen, which reduces the effective local cooling of electrons and permits T e to rise initially. The higher gas temperatures associated with further increases in solar activity cause enhancement of the heavier neutral constituents (O, He), which again increases the local electron cooling and results in a return to lower electron temperatures.