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 (|∇Ne/Ne|min-130 km) ionospheric conditions. Some theories for the generation of auroral roar require that the relationship between the electron plasma frequency (fpe) and the electron gyrofrequency (fce) in the source region is fpe2=(n2-1)fce2, 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. ¿ 1998 American Geophysical Union