[en] Consideration is given to the development and verification of global computer models of the F-region which simulate the interactions between physical processes in the ionosphere. The limitations of the physical models are discussed, focusing on the inputs to the ionospheric system such as magnetospheric electric field and auroral precipitation. The possibility of coupling ionospheric models with thermospheric and magnetospheric models is examined. 59 refs

[en] Theoretical line-of-sight velocities, as would be observed by the EISCAT radar, are computed for idealized models of plasma convection in the polar ionosphere. The calculations give the velocity as a function of range and Universal Time. For several variants of the Volland and Heelis convection models, how the maxima, minima and reversals of velocity depend on beam azimuth is examined. The analysis is designed to be applied to data from the UK-POLAR experiment, an example of which is shown

[en] A study of the reflection of Alfven waves at a horizontally inhomogeneous ionosphere has been carried out. In this study, the Alfven speed above the ionosphere is assumed to be uniform and the ionosphere is treated as a height-integrated conducting slab. Analytical and numerical results indicate that the horizontal nonuniformity of the ionospheric conductivity can lead to a rotation of the reflected wave fields and cause field-aligned currents that originate in the ionosphere. A strong conductivity nonuniformity in the direction perpendicular to the incident wave field, large Hall to Pedersen conductivity ratios, and low conductivity values lead to a large rotation of the reflected wave field in the range from a few to 40 degrees. The implications of the theoretical results for some ionospheric phenomena are presented, including comments on the Harang discontinuity and Sun-aligned arcs

[en] Recent experimental evidence has shown that Defense Meteorological Satellite Program (DMSP) polar orbiting spacecraft at 840 km can develop electric potentials as severe as -1,430 V while at high magnetic latitudes. The explore this charging region, an analysis of DMSP F6, F7, F8, and F9 satellite precipitating particle and ambient plasma measurement taken during periods of high, medium, and low solar flux is performed. One hundred eighty-four charging events ranging from -46 to -1,430 V are identified, and an extreme solar cycle dependence is found as charging is most frequent and severe during solar minimum. Satellite measurements and time-dependent ionospheric model (TDIM) output are used to determine the cause of the solar cycle dependence and to characterize the environments which generate and inhibit these potentials. The electron precipitation associated with various DMSP charging levels is analyzed; it is suggested that precipitating electrons as low as 2 to 3 keV may contribute to charging through higher-energy electrons make greater contributions. Secondary electron production due to incident electrons below 1 keV is shown to inhibit charging. The energetic electron fluxes shown to generate charging do not vary significantly over the solar cycle. Instead, DMSP ambient plasma data and TDIM generated results identify a variation in plasma density over 1 or more orders of magnitude as the cause of the solar cycle dependence, and an ambient plasma density of less than 10⁴ cm^-3 is found necessary for significant negative charging (≥100 V) to occur

[en] A time-dependent, three-dimensional, multi-ion numerical model of the global ionosphere was used to study the asymmetry in large-scale ionospheric features between the northern and southern hemispheres. The comparisons were done for June and December solstice conditions at solar maximum for quiet geomagnetic activity. In comparing the ionospheric densities in the northern and southern hemispheres for a given season, the authors found the following: (1) the winter hemispheres display the most marked universal time (UT) variations due to the displacement between the geomagnetic and geographic poles, (2) the summer high-latitude and equatorial densities in both hemispheres are morphologically similar. (3) In the winter hemispheres, the corresponding electron densities are again morphologically similar but can be very different quantitatively due to the different dipole offsets, (4) for a given season, the density difference between the northern and southern hemispheres displays a marked UT dependence, (5) the winter mid-latitude trough is the feature that exhibits the largest northern-southern hemisphere difference due to the different dipole tilts, and (6) the winter anomaly is present in the northern hemisphere at almost all UTs, while it is essentially absent in the southern hemisphere

[en] Simulations of the ionospheric model of Schunk et al. (1986) have been used for climatology and weather modeling. Steady state empirical models were used in the climatology model to provide plasma convection and particle precipitation patterns in the northern high-latitude region. The climatology model also depicts the ionospheric electron density and ion and electron temperatures for solar maximum, winter solstice, and strong geomagnetic activity conditions. The weather model describes the variations of ionospheric features during the solar cycle, seasonal changes, and geomagnetic activity. Prospects for future modeling are considered. 23 references

[en] Advances in all areas of ionospheric research are reviewed for the 1987-1990 time period. Consideration is given to the equatorial ionosphere, the midlatitude ionosphere and plasmasphere, the auroral ionosphere, the polar ionosphere and polar wind, ionospheric electrodynamic inputs, plasma waves and irregularities, active experiments, ionospheric forecasting, and coupling the ionosphere with other regions

[en] A three-dimensional time-dependent ionospheric model was used to study how electric field structures affect the polar F region. The electric field structures are represented by elongated Volland two-cell models whose dimensions range from tens to 1,000 km. These model structures are intended to represent the polar cap electric field for IMF B_z northward conditions. A statistical method is used to generate a set of these structures. Their electric field strength and polarity are varied in order to study the F region's dependence on this magnetospheric input. For electric field structures whose size and electric field strengths are consistent with B_z northward observations, several ionospheric dependencies were found. The n_mF₂ parameter, in general, decreased due to the presence of the higher electric fields associated with the structures. Decreases ranging from a few percent to a factor of 4 were obtained from the simulations. This decrease in N_mF₂ was further complicated by the initial N_mF₂ conditions, i.e., past history of the flux tube, being quite different when the structure is present. H_mF₂ in the vicinity of a structure can be raised or lowered by up to 100 km, however, this change is not uniquely dependent upon the vertical induced drift. The role of enhanced reaction rates due to elevated ion temperatures is very important. The F region becomes spatially very structured. The modulation of these structure in N_mF₂ is up to an order of magnitude in this study

[en] Ambient thermal electrons are found to be heated to temperatures as high as 10⁵K by the passage of a field-aligned beam of suprathermal electrons through the ionosphere at altitudes over 660 km. These secondary electrons will increase the proportion of 630 nm emission, caused by the primary electron precipitation, and change the secondary electron spectrum observed at lower altitudes from that expected on the basis of atmospheric collisions alone. (author)

[en] Observations by the suprathermal plasma analysers on GEOS 2 of a population of electrons over 50 eV which are confined to pitch angles near 90⁰ (a 'pancake' distribution) in the region of the magnetosphere where cold plasma, originating from the ionosphere, mixes with hot plasma-sheet particles, are reported. Data are presented of the energy dependence of the pancake distribution. The distributions are thought almost certainly to result from wave interaction. (UK)