The mode theory of ELF waves has been extended by Galejs to isotropic ionosphere models of an exponential conductivity-height profile, where the refractive index may be near unity at the lower boundary of the ionosphere. Recent experimental data of Bowhill on the lower nighttime ionosphere differ significantly from earlier estimates. The exponential ionosphere model is re-examined in view of these data. The calculated daytime attenuation rates were shown to agree with measurements of Jean, Murphy, Wait, and Wasmundt. The calculated nighttime attenuation rates are slightly higher than those measured by Chapman and Macario. The earth-to-ionosphere cavity resonance frequencies are computed using the average of day and night ionosphere parameters. They differ by less than ½ cps from the resonance frequencies measured by Balser and Wagner. The calculated Q factors of the cavity are approximately 4.5 in the frequency range between 10 and 30 cps. The computed terrestrial ELF noise spectrum exhibits a more rapid decay at higher frequencies than the measured spectrum. The wave form that results after passing the calculated noise spectrum through a bandpass filter exhibits an average frequency consistent with measurements of König.