<1> By using a large sample of data on sudden phase anomalies (SPA) of VLF signals (frequencies 11.9 and 13.6 kHz), a model of winter and summer electron density profiles N(h) of the near-noon lower ionosphere (45-75 km) for the conditions of sudden ionospheric disturbances (SIDs) has been developed. The main subject of the study has been parameter b, which characterizes the relation between the electron density N and ionization rate q. The dependencies of b on height h and latitude θ (in the interval 35o S to 55o N) have been found for summer and winter. The magnitudes of parameter b obtained by analyzing SPA have been compared with its values calculated by using a theoretical detailed model of ion chemistry of the lower ionosphere and also the global N(h) model of the ionosphere for quiet conditions. Below 65 km the results are consistent. The discrepancies in the magnitudes of b occur for middle latitudes 30o-55oN, summer conditions, and heights of 70-75 km. The changes in the electron density profiles during SIDs deduced from the SPA data for these conditions are several times higher than theoretical estimates. Considerable latitudinal variations in parameter b and N(h) profile during SID have been found to occur only in summer at the heights indicated above and latitudes 0o-30oN.