Asymmetry of the longitudinal effect in the low-latitude ionosphere Original Research Article

From the Intercosmos-19 satellite topside sounding data the dependence of the low-latitude nighttime F2 layer heights on latitude and longitude was analysed and hmF2 maps were constructed. Longitudinal variations of the F2 layer heights with periods of 75° to 100° and average amplitudes of 50 to 70 km, occasionally greater than 100 km were found. Variations are stronger in the winter hemispheres, generally more pronounced in the Southern hemisphere than in the Northern one. Longitudinal variations of hmF2 are so strong and their extrema at certain longitudes are so stable that they are clearly seen even in maps established after averaging a great number of measurements at different times. Observed North/South asymmetry in hmF2 combined with strong longitudinal variations suggest corresponding patterns in the thermospheric winds.