Ionospheric structure and the generation of auroral roar

Ionospheric electron density data from the Sondrestrom incoherent scatter r adar (ISR) have been used to characterize the structure of the F region ion osphere during ground-based LF/MF/HF receiver observations of natural ionos pheric radio emissions known as auroral roar. In five out of six cases, the F region ionosphere has significant horizontal N-e gradient scale lengths (\Delta N-e/N-e\(-1)(min) < 120 km, measured with 23-137 km spatial resolut ion). In three of these cases, localized F region auroral ionospheric cavit ies, with horizontal scales similar to 50 km, are observed. In one of six c ases, the ionosphere lacks either of these features, and a laminar, mostly unstructured, F region is observed. The data suggest that auroral roar even ts may occur for a range of large-scale (>30 km) ionospheric conditions. So me theories for the generation of auroral roar require that the relationshi p between the electron plasma frequency (fp,) and the electron gyrofrequenc y (f(ce)) in the source region is f(pe)(2) = (n(2) - 1)f(ce)(2), where n is the harmonic number of the observed emission. Comparisons between observed auroral roar emission frequencies, ISR observations of electron density, a nd 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 a nd in all 3 cases of ISR elevation scans capable of measuring a source loca ted directly overhead.