Ionospheric variations over Indian low latitudes close to the equator and comparison with IRI-2012

In this paper, we analyze daytime observations of the critical frequencies of the F2 (foF2) and F3 (foF3) layers based on ionosonde observations made from Indian low latitudes close to the magnetic equator and study their local time, seasonal, planetary-scale variations (including the solar rotation effect), and solar activity dependence. Given the occurrence of the F3 layer, which has remarkable local time, seasonal and solar activity dependences, variations in foF2 have been evaluated. Local time variations in foF2 and foF3 show noon "bite-out" in all seasons and in all solar activity conditions, which are attributed to vertically upward plasma transport by the zonal electric field and meridional neutral wind. Comparison of observed foF2 with those of the IRI-2012 model clearly shows that the model values are always higher than observed values and the largest difference is observed during noontime owing to the noon bite-out phenomenon. Peak frequency of the F layer (foF2 / foF3), however, is found to have better agreement with IRI-2012 model. Seasonal variations of foF2 and foF3 show stronger asymmetry at the solstices than at the equinoxes. The strong asymmetry at the solstice is attributed to the asymmetry in the meridional neutral wind with a secondary contribution from E × B drifts, and the relatively weak asymmetry observed at the equinox is attributed to the asymmetry in E × B drifts. Variations in foF2 and foF3 with solar flux clearly show the saturation effect when F10.7 exceeds ~ 120 sfu, which is different from that of the mid-latitudes. Irrespective of solar flux, both foF2 and foF3 in summer, however, are found to be remarkably lower than those observed in other seasons. Variations in foF2 show dominant periods of ~ 27, ~ 16 and ~ 6 days. Intriguingly, amplitudes of ~ 27-day variations in foF2 are found to be maximum in low solar activity (LSA), moderate in medium solar activity (MSA) and minimum in high solar activity (HSA), while the amplitudes of ~ 27-day variations in F10.7 are minimum in LSA, moderate in MSA and maximum in HSA. These results are presented and discussed in light of current observational and model-based knowledge on the variations of low-latitude foF2 and foF3.