Ionospheric electron density data from the Sondrestrom incoherent scatter radar (ISR) have been used to characterize the structure of the F region ionosphere during ground-based LF/MF/HF receiver observations of natural ionospheric radio emissions known as auroral roar. In five out of six cases, the F region ionosphere has significant horizontal Ne gradient scale lengths ( , measured with 23–137 km spatial resolution). In three of these cases, localized F region auroral ionospheric cavities, with horizontal scales ∼50 km, are observed. In one of six cases, the ionosphere lacks either of these features, and a laminar, mostly unstructured, F region is observed. The data suggest that auroral roar events may occur for a range of large-scale (>30 km) ionospheric conditions. Some theories for the generation of auroral roar require that the relationship between the electron plasma frequency (ƒpe) and the electron gyrofrequency (ƒce) in the source region is , where n is the harmonic number of the observed emission. Comparisons between observed auroral roar emission frequencies, ISR observations of electron density, and the IGRF model for the magnetic field show that this frequency-matching condition holds somewhere in the ionosphere in 16 out of 18 cases studied and in all 3 cases of ISR elevation scans capable of measuring a source located directly overhead.