Lower ionosphere at solar minimum

Two Nike-Apache rockets were launched in 1964 to measure: (a) positive ion density (N+) with an altitude resolution of approximately 10 meters by use of a modified Gerdien condenser, (b) electron density by radio-propagation techniques, and (c) the optical depth of solar radiation absorbed in the 60–120 km region with a photoelectron retarding potential analyzer. The flights took place at a time when the intensities of important portions of the solar spectrum were being measured simultaneously from a satellite. The simultaneity of all these data and the high altitude resolution of the charged particle density profiles permits us to identify several regions between 65 and 120 km and to associate them with different portions of the solar spectrum and with different loss mechanisms. The average N+ in the D region (65–83 km) is found to be 103cm−3, an order of magnitude less than reported by investigators who used experiments that, unlike ours, require assumptions about other ionic parameters to derive N+. The regions 83–88 km and 88–93 km are sequentially ionized by 2–8 A X radiation and the C VI line at 33.7 A that produce O2+ and N2+ ions. These ions are transformed into NO+ by processes involving charge transfer and/or ion-atom interchange (N2+ + O2 → O2+ + N2 → NO+ + NO). From our results we compute the effective loss rate between 83 and 93 km to be approximately 2×10−8 cm3 sec−1, which quite likely represents the dissociative recombination rate for NO+. The 95–115 km region is ionized principally by extreme ultraviolet radiation leaving O2+ as one of the two dominant ions. Though never dominant, 40–75 A X radiation is an important ionizing source which indirectly produces some NO+ ions above 95 km through the process N2+ + O → NO+ + N. Our computed effective dissociative recombination rate between 95 and 115 km is about 1.8×10−7 cm3 sec−1. It is suggested that this value is higher than that computed for the region below 90 km because above 95 km the ionic content is richer in O2+.