Fine structure of foF2 longitudinal distribution in the night-time low-latitude ionosphere derived from intercosmos-19 topside sounding data

Abstract

From the Intercosmos-19 satellite topside sounding data, a longitudinal fine structure in the distribution of foF2 in the night-time low-latitude ionosphere was found and analysed for quiet geomagnetic conditions of all seasons at solar maximum. Distinct from station-based investigations the satellite orbit allows to study at fixed local time the longitudinal structure in much detail. It was found that the longitudinal patterns of peak electron density have an almost wavelike character with periods of 75° to 100° in longitude. Extrema of this structure tend to keep their positions at fixed longitudes for all local times and seasons, while their amplitudes vary on average from 2 to 3 MHz at the magnetic equator and from 3 to 4 MHz at the anomaly crests, the sharpest changes being found near the longitude 0° where foF2 differences can exceed 8 MHz over 50° longitude. The deepest minimum of foF2 occurs near 330°. In December solstice the amplitudes are somewhat smaller than in June solstice and the minimum near 330° is less pronounced. Polewards of the crests this wavelike structure gradually disappears. The mechanisms responsible for this phenomenon are yet unknown as influence of the geomagnetic field can explain only the coarse pattern of longitudinal changes of the equatorial anomaly but not its fine structure. As one cause of this phenomenon a multi-mode structure of the thermospheric winds, temperature and composition of the thermosphere, as well as of electric field is proposed. In the numerical maps (URSI) option) that are reproduced in the International Reference Ionosphere (IRI) such structures are almost smoothed out.