Ionospheric electron density profiles at sunrise-sunset

In order to improve its representation of the dependence on time and space of the ionospheric parameters, the International Reference Ionosphere ought to take account of realistic sunrise and sunset conditions in the upper atmosphere. Such input is needed for quite a few parameters for which only day and night values were taken as input in the present IRI. Of the 24 hours of a day, true nighttime comprises a fraction of 37% at an altitude of 300 km and only 26% at 1000 km. In order to demarcate the day/night/day transition periods, the present IRI proposes solar zenith angles of 98° to 120°, depending on the altitude.Electron density profiles, obtained during these periods, have been studied with two data sources: 10 vertical-incidence sounding data observed during the meridional voyages of the research vessel “Akademik Korolev” in the Pacific Ocean; 2° data observed at the South Pole. It is shown that the height of the turning point in the sub-peak F2-layer profile and also the corresponding minimum scale height appear to be independent of latitude, season and index of geomagnetic activity. A method is discussed by which the IRI electron density profiles might be improved, in particular during these hours.