Ion-Chemical Model of the Lower Ionosphere

A theoretical, ion-chemical model of the ionised part of the mesosphere (D-Region) is to be developed. The calculations are not intended to compete with full complex atmospheric models which compute not only ions and electrons, but also self-consistently neutrals and dynamics. The main purpose is to have available a user-friendly tool with which the large number of empirical ionospheric data available at the institute can be assessed. Notably the model calculations are to provide skeleton functions onto which corrections based on the empirical data can be applied, thus leading to new and realistic models of the most important bulk parameters of the lower ionopshere.

The mesosphere, i.e. the region above the stratosphere up to about 90 km altitude of the Earth`s atmosphere is only accessible for in-situ measurements by sounding rockets. By the nature of such measurements they are rare and in consequence this atmospheric region still poses a number of unresolved questions. The ionised part of this altitude range is the ionospheric D-region. The chemistry governing the mesosphere (both of neutrals and ions) is very much a topic of modern investigations, not least because of the strong influence of trace gases at those heights. The data base of in-situ measurements is increasing, and with each new data set new questions of interpretation arise. The need for more insight into the underlying ion chemistry is obvious. The objective of this project was to establish a better, more representative, ion-chemical model of the lower ionosphere to provide realistic start values for empirical ionospheric models and to form a reference for case studies of electron density and other parameters in the ionosphere, such as the ratio between large cluster ions and small molecular ions, and the ratio between densities of negative ions and electrons. In modelling of the ion chemistry, the neutral atmosphere is generally taken as input varying with time and location; ionisation sources vary with solar activity. A steady-state approach for the ion chemistry is justified in the lower ionosphere, where ion-chemical time constants are short compared to transport effects. The new model merges features of the models existing at the Space Research Institute in Graz, and the SIC model by the Sodankylä Geophysical Observatory in Finland, and enhances them by the latest available inputs for bulk atmospheric parameters and ionisation sources. It became obvious that the reference atmosphere CIRA-86 only insufficiently describes the true variation of pressure, temperature and density. Using the atmospheric data gathered at the Andøya Rocket Range and the co- located Lidar installation Alomar a correction to CIRA was established which differs by up to 20 % in density or pressure and up to 15 K in temperature from CIRA for 70 degree northern latitude. A literature search for data on nitric oxide (NO), the most important trace gas for the formation of the D-region, and for atomic oxygen, the second most important minor constituent in this context, was made, and the data imported into the model. Since the aim was to eventually improve empirical models, a survey and a re-investigation of the available measured ionospheric data, notably electron densities, was made. In that process several more electron density profiles were found in the literature and some of the data already in the data base were corrected. The data bank based on wave propagation methods increased by 44 to 322 profiles, and a set of 450 probe profiles was added to the data base. The improved data volume was investigated for residual variations. An asymmetry was found between pre- and post-noon electron densities, particularly in the height region between 70 and 80 km, i.e. where nitric oxide and the Lyman-Alpha line of the solar spectrum dominate the ion production. This asymmetry needs to be investigated further, notably in terms of a diurnal asymmetry of neutral nitric oxide.