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Detailed computer calculations of the electron, O2+, and NO+ concentrations are provided for the E region on a diurnal basis. The present work contains the first continuous solution of these concentrations during sunrise and sunset. Comparison is made between this model and observations, particularly new results at sunrise and sunset. The model is compatible with the experimental data within a factor of 2, although some larger discrepancies occur. The nitric oxide distribution of Barth for altitudes 90–120 km appears to provide for the proper amount of conversion of O2+ into NO+ via O2+ → NO+ → NO+ + O2 and for an important twilight ionization source, H Ly α + NO → NO+ + e. However, at 85 km, the model requires a lower concentration of nitric oxide than found by Barth in order to be consistent with the observed charged particle concentrations. The process N(²D) + O2 → NO + O is considered as a major NO‐producing process in the E region. The most abundant atomic nitrogen species, N (4S), appears to play no role in the E region, except perhaps at 130 to 140 km. An upper limit of about 108 cm−3 is implied for atomic nitrogen. The calculations include H Ly β as the main nighttime ionization source. It is stressed that the mean recombination coefficient of the E region appears to be larger at night than by day and largest at twilight.

A one-dimensional time-dependent model of the ionosphere has been developed and applied to the study of a metallic ion sporadic-E layer observed in the Aladdin 1 experiment carried out at Eglin AFB, Florida, 20 November 1970. The model develops the molecular ion background ionosphere using a dynamic photochemical calculation from noon to a time near model sunset. A representative metallic ion altitude profile is then introduced, the divergence terms included in the continuity equations, and the integration carried forward. Introducing an ad hoc constant electric field of 2 mv m -• directed to the south, the model metallic ion sporadic-E layer forms at the proper altitude and reaches the measured peak density in about a half hour. Changing the initial metallic ion profile changes the time to reach peak density and the degree of asymmetry of the layer, but the layer altitude is determined asymptotically by the location of the convergent node of the vertical ion velocity profile. The background ionospheric density calculated with the model agrees within experimental error with the experimental profile. The calculations support the hypothesis that midlatitude sporadic-E layers are caused by neutral-wind-induced compression of metallic ions resulting from meteoric ablation in the lower E region. 3 of the model [Keneshea and MacLeo& 1970; MacLeod et al., 1973]. The terms in the second bracket which depend on the electric field have the same form as a neutral wind u'--E/O•, suggesting that their main effect on the motion will come from the upper E and F regions as p•--) 0. It will be seen below that these terms can also produce observable effects in the lower E region. The ion continuity equation. The general continuity equation for the ith ion species may be written allilar= Qi-Li + • ' (D•7rti-lli¾i) (6) where Q• and L• are the general source and loss terms for species i and depend on the n i number NUMERICAL MODELLING 373

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