Knowledge of the ionospheric electron density is essential for a wide range of applications, e.g., radio and telecommunications, satellite tracking, and Earth observation from space. Considerable efforts have, therefore, been concentrated on modeling this ionospheric parameter. Several models are described on the following pages including the phenomenological Chiu model, the Bent model that has been used extensively for satellite tracking, the semi-empirical SLIM model based on theoretically obtained grid values, and the recent FAIM model that uses the Chiu formalism together with the SLIM results. The International Reference Ionosphere (IRI) is probably the most mature of these models, having undergone more than two decades of scrutiny and improvement.
The ionospheric electron density profile exhibits several peaks with the F2-peak being the largest and most important. Using spherical harmonics world maps have been developed for the F2-peak critical frequency foF2; the F2-peak electron density is linearly related to foF2 squared. Similar maps have been established for the propagation factor M(3000)F2, which is related to the height of the F2-peak. This listing includes the widely used set of coefficients recommended by the International Radio Consultative Committee (CCIR), the newly proposed set of the International Union of Radio Science (URSI), and the mission-specific maps obtained by the Japanese Ionospheric Sounding Satellite-b (ISS-b) during 1978-1979. MINIMUF and IONCAP are software packages designed specifically for radio propagation purposes. They are available from NOAA's World Data Center A for Solar-Terrestrial Physics in Boulder, Colorado.
Fewer mostly mission-specific models have been developed for the electron temperature, ion composition (relative ion densities in percent) and the ion drift, the latter being closely related to the forcing electric field. The International Reference Ionosphere, the most complete representation of the ionosphere, includes models of the ion temperature, electron temperature, ion composition, and ion drift.
At present, almost all empirical models of ionospheric parameters are limited to non-auroral, magnetically quiet conditions. Major efforts are underway to extend ionospheric predictability beyond these limitations. A promising venue seems to be the inclusion of real-time data from the newly developed automatically recording and scaling ionosondes. These and other measurement techniques are discussed in a report published by NSSDC: D. Bilitza, The Worldwide Ionospheric Data Base, NSSDC 89-03, Greenbelt, Maryland, 1989. The ionospheric compendium includes also a chapter on the present status of ionospheric models.
At high latitudes, the plasma density and temperature are strongly affected by electric fields and by particle precipitation. Empirical modeling of the electric field and the electron and ion precipitation has shown great progress over the last decade as documented by the entries in this report. The auroral and polar zones are the regions of close coupling between ionosphere and magnetosphere. Both the electric (convection) field and the precipitating particles are of magnetospheric origin. The models are listed here rather than in the magnetospheric section because they were all established based on measurements at ionospheric altitudes by polar-orbiting satellites.
The Volland, Heelis, and Utah electric field models are semi-empirical in the sense that theoretically derived expressions are adapted to observed features. Empirical models have been developed from ground-based geomagnetic measurements (IZMIR model), from incoherent scatter data (Millstone Hill model), and from Dynamic Explorer (DE) satellite measurements (Heppner-Maynard-Rich model).
The AFGL Electron Precipitation model and Ion Precipitation model for the flux and energy of precipitating particles at auroral latitudes are based on DMSP satellite measurements at about 800 km altitude. The Rice model was established with Atmospheric Explorer-C (AE-C) and Atmospheric Explorer-D (AE-D) data.