Improved observations in the Schumann resonance frequency range have expanded interest in improved models for the Earth-ionosphere waveguide, particularly models that incorporate day-night asymmetry. Aeronomical data are used here to construct daytime and nighttime analytic conductivity models of the lower characteristic layer, one of two vertically separated regions governing ELF propagation in the waveguide. The models have the form of double-exponential functions that represent the "knee"-like transition from ion- to electron-dominated conductivity in a physically realistic manner. On the basis of these profiles, analytic approximations of the lower complex characteristic altitude are obtained for the frequency range 3--100 Hz. The approximations are accurate and simple to use for parametric analysis.