Topside ionospheric scale height analysis and modelling based on radio occultation measurements

The knowledge of the scale height in the topside ionosphere region remains rather poor due to the insufficient observations carried so far. To advance this knowledge, presented here is a new method of retrieving the topside ionospheric scale height based on radio occultation observations onboard low-earth-orbiting satellites. The scale height, well known for its dependence on the temperatures and masses of the ionospheric constituents, understandably experiences large spatial and temporal variability. With the help of the CHAMP satellite's occultation experiment, analysed is the scale height behaviour with respect to solar and geomagnetic activity, local time, season, longitude and latitude. The expected strong dependence on temperature has been confirmed; however, it has been found that this dependence is not straightforward but more complex and clearly affected by other factors. For example, while the daytime scale height values increase at higher solar activity, the night-time values do not show such a trend. The seasonal dependence proved to be strong with summer-time values significantly higher than winter-time numbers. Also, there is no common pattern for the diurnal variations: sometimes daytime values are higher, sometimes the night-time values dominate; large differences are detected from season to season and from latitude to latitude. Generally, the scale increases at higher latitudes, although a few differences do exist. No major longitudinal and hemispheric differences have been detected so far. Based on the accumulated data, a first attempt has been made to empirically simulate the scale height value at 425 km altitude; as input parameters, the model respects the local time, latitude, and season. The scale height model can be implemented into the electron density profile retrieval procedure by delivering an improved initial guess.