Bottomside electron density (Ne-h) profiles below the F2-peak, during midday (10-14 h) are analyzed using modern digital ionosonde observations at New Delhi, for the period from January 2003 to December 2003, pertaining to moderate solar activity (MSA). Each individual profile is normalized with respect to the peak height and density (hmF2, NmF2) of the F2-region. These profiles are compared with those obtained from the International Reference Ionosphere (IRI-2001) model. using both the options namely: Gulyaeva’s model and B0 Tab. option. The study reveals that during summer and equinox, the IRI model with B0 Tab. option in general, produces better agreement with the observed median profiles, while the IRI predictions using Gulyaeva’s option, overestimate the electron density distribution at all the heights below the F2-peak, the IRI (Gulyaeva) model predicted electron densities are larger by about 100 to 125 % at heights around 100 km below the F2-peak. However, during winter, in general, the IRI model, using both the options, reveals shows fairly good agreement with the observed median profiles. The said discrepancies are examined in terms of profile shape parameters, in particular for Gulyaeva option, which uses height of the half peak electron density of the F2-layer (h0.5) for building up of the bottomside profile in the IRI model.

Although, the recommended option in IRI for low latitude is that of Gulyaeva, but it does not reproduce observed profiles well, rather, the profiles predicted by IRI with Gulyaeva’s option are too thick especially during summer and equinox (Fig.6.6). Examining the h0.5 parameter from the observed and IRI model, it is concluded that the h0.5 parameter, is not well suited particularly during summer and equinox for daytime conditions.