LowerE-region ion concentrations measured at a time of declining solar activity

A positive ion mass spectrometer of the Bennett type, mounted in an ejectable payload package, was carried to 114 km above Wallops Island, Virginia, at 1330 EST in May 1962. Nitric oxide and molecular oxygen ions were the predominant species measured in the 87- to 114-km altitude range, and near 100 km they were approximately equal in concentration. Above 100 km NO+ increased in concentration to 113 km where its density was 105 ions/cm³, and the concentration of O2+ was 4 × 104 ions/cm³. The other ions observed during the flight were 24+ and 28+. The ion 28+, identified as molecular nitrogen and/or silicon, was observed between 103 and 114 km, and reached a maximum concentration of 104 ions/cm³ at 113 km. The ion 24+, identified as magnesium, was observed in two distinct layers, the lower near 93 km and the upper at 113 km; the maximum concentration of this ion was 1.5 × 10³ ions/cm³. The total ion concentration varied from approximately 4 × 10³ ions/cm³ at 90 km to 105 ions/cm³ at 110 km. The fact that NO+ and O2+ are of approximately equal concentration in the lower E-region compares favorably with ion concentration data obtained near solar minimum by other investigators; the significance of this result is examined in the light of current theories that treat the relative importance of NO+ and O2+ and the removal of N2+ ions in this region.