Auroral kilometric radiation (AKR) observations from the ISIS 1 topside sounder receiver were compared with visual auroral observations from the network of Alaskan all-sky camera stations. The goal was to relate AKR source region encounters to specific auroral forms on the same magnetic field line. Thirty-eight simultaneous data sets were identified and analyzed. In general, intense AKR was associated with bright auroral arcs, and conditions of weak or no AKR corresponded to times when either no aurora or only a faint arc or weak diffuse aurora was observed. Five cases, when both intense AKR and bright visual aurora were present, were analyzed in detail. Complete electron density Ne contours, from the satellite altitude down to the F region ionization peak, were obtained along N-S traversals of the AKR source region. In addition, the ISIS 1 orbital tracks were projected down the magnetic field lines to the auroral altitude and compared to auroral features on a map derived from the all-sky camera images. Density cavities (regions where Ne < 100 cm−3) were encountered on each of these passes. In one case the cavity extended in altitude down to just below 2000 km. The latitudinal extent of these cavities ranged from 3° to almost 30°. A variety of situations relating AKR to the visual aurora were identified, and both spatial (within about 100 km at the satellite altitude) and temporal (within about l min) correlations were identified. In addition to confirming that a density cavity is a necessary but not sufficient condition for the generation of AKR, the present results indicate that even the combination of a density cavity on an auroral-arc field line is not always sufficient for AKR generation. Thus additional constraints must be satisfied in order to generate intense AKR from density cavities. These constraints most likely involve the characteristics of the energetic electron distribution function and possibly also latitudinal Ne gradients.
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