ISIS 1 topside sounder receiver observations that reveal examples of O mode auroral kilometric radiation (AKR) are presented. They correspond to locations outside the low-density source region of intense AKR X mode emission. The propagation modes are identified by comparing the natural radiation wave cutoffs with the local resonant and wave cutoff phenomena stimulated by the sounder transmitter. The O mode AKR is the dominant emission in these regions of relatively high electron density, but it is considerably weaker than the intense X mode AKR observed to emanate from low-density cavities above the auroral regions. In addition to the fundamental O mode, second and third harmonic bands of radiation have also been detected. These harmonics associated with O mode AKR are less intense than the harmonics associated with X mode AKR. It is difficult to explain the variety of harmonic AKR observations (X as well as O mode) on the basis of present AKR theories.
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