Molecular ion concentration around a dawn-dusk auroral-zone satellite orbit

Molecular ion concentration n(O2+ + NO+) has been obtained from a curve-fitting technique applied to characteristic curves of a retarding potential analyzer carried on an oriented satellite for four days in April 1962. Data were obtained continuously around the orbit, whose orientation was such that most data were obtained at local times near dawn and dusk. Apogee (516 km) and perigee (210 km) occurred on the dawn and dusk sides of the orbit, respectively. At or near the magnetic equator on the dusk side of the orbit, a trough in n(O2+ + NO+) developed with a trough-to-shoulder ratio that varied between one-half and one-fifth. The ratio n(O+)/n(O2+ + NO+) tended to remain constant with universal time over the portion of the orbit near perigee (local time ≃1700 hr). The concentrations n(O+) and n(O2+ + NO+) varied individually by as much as a factor of 2–3. The time average value of n(O2+ + NO+) was found to be 4 × 104 cm−3 from 15 to 45° N invariant latitude at a local time of 1700 hr. On some orbits there is indication of a significant decrease of molecular ion concentration in the northern auroral zone when compared to latitudes south of the zone. The molecular and total ion concentrations near perigee exhibited a universal time variation with a 24-hour period, which may either be the result of a longitudinal variation in these quantities or the result of some broad-latitude universal-time variation.