Simultaneous radio and optical observations of auroral structures: Implications for AKR beaming

Polar spacecraft observations of auroral kilometric radiation (AKR) spectra, combined with simultaneous optical images of the auroral oval at the source foot points, show a strong correlation with discrete auroral features. We discuss several examples of distinctive "V"-shaped profiles in time-frequency AKR spectra for which the time of nadir passage (bottom of V) coincides with the spacecraft crossing field lines connected to discrete auroral arcs. These results support models of AKR emission originating in density cavities with radiation initially confined to a "tangent plane" but strongly refracted upward in a frequency-dependent manner. The inferred spatial distributions of AKR sources are closely coincident with field lines connecting discrete auroral arcs, indicating that discrete arcs are highly correlated with robust AKR emission on the same field lines, as first suggested by Gurnett (1974) several decades ago.