Unlike the conventional representations of the vertical electron density profile N(h)F2 of the F2 layer in the ionosphere around peak density NmF2 by a single-maximum 'pointed-peak' function, this paper introduces the Peak Density Thickness (PDT) formalism, which allows the possibility for the plasma density at the peak to remain constant within an altitude interval χ. Several independent observations support such a 'broad-peak' or 'flat-nose' concept, including simultaneous measurements of the sub-peak and super-peak density profiles using different radar techniques. In particular, profile measurements obtained at the mid-latitude MIT Haystack observatory by the Millstone Hill Incoherent Scatter Radar (MH-ISR) reveal the presence and diurnal variation of PDT, ranging from ~ 0 km at nighttime to tens of km at midday. The PDT measurements by MH-ISR were coordinated with the Digisonde Portable Sounder DPS4D soundings of the sub-peak ionosphere up to hmF2. Remarkable agreement of these independent measurements of the lower boundary of the χ interval, hmF2, and the peak density, NmF2, is observed compared to MH-ISR when its ion-line acoustic resonance scattering specification of N(h) is complemented with an accurate F2 peak plasma-line ordinary Langmuir wave measurement of NmF2 for the profile calibration. PDT remains highest during the afternoon hours before starting to decrease after sunset. Case studies demonstrate how PDT can be estimated from the Digisonde bottomside thickness parameter B0. Allowing for the flat-nose section χ in the F-region profile formalism will improve the accuracy of the topside specification deduced from ground-based ionosonde measurements.