Seasonal variation of theF₁-region ion composition

Thomson scatter observations at a wavelength of 23 cm are described that supplement earlier ionospheric studies made at the Millstone Hill Radar of the region 130–230 km. By making certain reasonable assumptions about the altitude dependence of the electron and ion temperatures, it has been possible to recover from the observations the ratio of atomic ions, (O+) to molecular ions (O2+ or NO+) in this region. A marked seasonal variation is found with O+ extending to lower altitudes in winter than in summer. The results suggest a seasonal variation of the abundance of neutral atomic oxygen that has not yet been detected in rocket-borne ion mass spectrometer measurements.