We have studied the dynamics of the nighttime high‐latitude F region with special emphasis on the formation of the electron density trough region which lies equatorward of the auroral oval. It is found that the absence of photoionization together with ordinary ionic recombination and slow plasma convection velocity can give a deep trough over a period of many hours. However, the normal global pattern of electric fields has regions of plasma convection sufficiently rapid to affect the rate of O+ + N2 reactions and to speed the rate of ionospheric decay. In addition, the escape of thermal plasma via the polar wind as well as N2 vibrational excitation and enhanced N2 densities act to deplete the ionosphere. In combination these destructive processes can readily account for the great variety of troughs found by experimentation. Thus it appears that there is no single cause for the observed troughs but that at various times, different processes act together to create density depressions of substantial magnitude.
We have obtained solutions of the coupled continuity, momentum, and energy equations for NO+, O2+, and O+ ions for conditions appropriate to the daytime high‐latitude E and F regions. Owing to the rapid increase of the reaction O+ + N2 → NO+ + N with ion energy, high‐latitude electric fields and consequent E⊥ × B drifts deplete O+ in favor of NO+. For electric field strengths less than about 10 mV m−1 the depletion of O+ is small, and the altitude profiles of ion density are similar to those found at mid‐latitudes. However, for moderate electric field strengths (∼50 mV m−1), NO+ is substantially increased in relation to O+ and becomes an important ion throughout the F region. For these conditions the electron density has a tendency to become nearly constant with altitude in the range 160–360 km. For large electric fields (∼200 mV m−1), NO+ completely dominates the ion composition to at least 600 km, decreasing at high altitudes with a diffusive equilibrium scale height. Since the overall F region electron density decreases markedly with increasing electric field strength, it appears that high‐latitude, daytime electron density troughs are directly related to the presence of ionospheric electric fields. In addition, since increases in the N2 density or the N2 vibrational temperature also affect ion composition and electron densities in a manner similar to that of electric fields, the observed daytime troughs may arise from both processes acting simultaneously.
Abstract. We combined a simple plasma convection model .th8n ionospheric-atmospheric composition model in order to ~dY the high-latitude winter F region at solar minimum for loW magnetic activity. Our numerical study produced time dependent three-dimensional ion density distributions for the ionS NO+: 0,+, N 2 +, 0+, N+, and He+. We covered the highlatitude ionosphere above 54°N magnetic latitude and at altitudes between 160 and 800 km for a time period of one completA! day. The main result we obtained was that high-latitude ionOSpheric features, such as the 'main trough,' the 'ionization bole,' the 'tongue of ionization,' the 'aurorally produced ionization peaks,' and the 'universal time effects,' are a natural consequence of the competition between the various chemical and tranSport processes known to be operating in the high-latitude ionosphere. In addition, we found that (1) the F region peak electron density at a given location and local time can vary by more than an order of magnitude, owing to the UT effect that results from the displacement between the geomagnetic and geographic poles; (2) a wide range of ion compositions can occur in the polar F region at different locations and times; (3) the minimum value for the electron density in the main trough is sensitive to nocturnal maintenance processes; (4) the depth and longitudinal extent of the main trough exhibit a significant UT dependence; (5) the way the auroral oval is positioned relative to the plasma convection pattern has an appreciable effect on the magnetic local time 8ltent of the main trough; (6) the spatial extent, depth, and location of the polar ionization hole are UT dependent; (7) the level of ion production in the morning sector of the auroral oval has an appreciable effect on the location and spatial extent of the polar ionization hole; and (8) in the polar hole the F region peak electron density is below 300 km, and at 300 km, diffusion is a very important process for both 0+ and NO+. Contrary to the suggestion based on an analysis of AE-C satellite data obtained in the polar hole that the concentration of NO+ ions is chemically controlled, we find diffusion to be the dominant process at 3OOkm.
We have extended our high-latitude, ionospheric, dynamic mo•el t• include N'+ in addition $o the ions NO-, Oh-, N• , and 0 ß The ion He was also include• but a•titude profiles of this ion were obtained from our previous polar wind study. We have further improved our model by updating the various chemical reaction rates and by including the latest solar EUV fluxes measured by the Atmosphere Explorer satellites, the most recent MSIS model of the neutral atmosphere (N?, 02, O, and lie) and the latest empirical model •f atomic nitrogen. The improved model was used to study the solar cycle, seasonal, and geomagnetic activity variations of the daytime high-latitude F layer. Both zonal and meridional convection electric fields were considered. Without. allowance for electric fields the peak 0 + and N + densities varied by an order of magnitude and the altitudes of the peaks varied by about 100 km over the range of geophysical conditions studied. Convection electric fields can also g•oduce $bout an order of magnitude change in the and N densities. These electric field induced changes could either assist or oppose the solar cycle, seasonal, and geomagnetic activity variations depending $n the ionospheric conditions. In general, N was the second most abundant ion in + the upper F region, but Shere were cases when lie + was more abundant than N even though He was in a state $f outflow. Also, we speculate that at times, N can be the dominant ion in the upper F region. 1967; Bailey+and Moffett, 1972]. At high latitudes, N is influenced by diffusion, major ion drag, electrodynamic drifts, and photochemical processes. In this study, the various transport processes were described by the equations presented by Schunk [1977], while the photochemical model was based on the processes found to be important at mid-latitudes. The photochemical+processes included are production due to the He + N reaction and photoionization of N and N 2 and lo•s due to reactions of N with 02 and NO. 2. Atmosphere-Ionosphere Model T e situstion we investigated was the behavior of 0 • and N-ions in a convecting daytime F layer for a range of geophysical conditions covering solar cycle, seasonal, and geomagnetic variations ß Basic Model Our basic model, including transport equations and ion chemistry, is given by Schunk and Walker [1973], while the extension of this model to include various high-latitude effects, such as the influence of convection electric fields, is given by Schunk et al. [1975, 1976]. In our model calculations, we follow a field tube of plasma as it convects across the dayside polar cap through a stationar% neutral armssphere. Altitude profiles of NO-, 0•-, and 0 density are z obtained by solving the appropriate continuity, momentum, and energy equations including the effects of both meridional an• zonal electric fields. The molecular ion N• is also considered, but it is assume• to be in chemical equilibrium at all altitudes (a valid assumption during the day). Improvements to the Basic Model The main improvement to our high-latitude io...
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