Abstract. High resolution electron density profiles (Ne) measured with the Arecibo (18.4 N, 66.7 W), Incoherent Scatter radar (I. S.) are used to obtain the bottomside shape parameters B0, B1 for a solar maximum period (1989–90). Median values of these parameters are compared with those obtained from the IRI-2001 model. It is observed that during summer, the IRI values agree fairly well with the Arecibo values, though the numbers are somewhat larger during the daytime. Discrepancies occur during winter and equinox, when the IRI underestimates B0 for the local times from about 12:00 LT to about 20:00 LT. Furthermore, the IRI model tends to generally overestimate B1 at all local times. At Arecibo, B0 increases by about 50%, and B1 decreases by about 30% from solar minimum to solar maximum.

Key words. Ionosphere (equational ionosphere; modeling and forecasting)