Observations of auroral kilometric radiation (AKR) are made by ISIS 1 in the source region. The radiation is found to be generated in the extraordinary mode just above the local cut-off frequency and to emanate nearly perpendicular to the magnetic field. It occurs within local depletions of electron density, where the ratio of plasma frequency to cyclotron frequency is less than 0.2. The density depletion is restricted to altitudes above about 2000 km, and the upper AKR frequency limit corresponds to the extraordinary cutoff frequency at this altitude. AKR is observed from ISIS 1 above the nighttime auroral zone over a wider extent in longitude than in latitude with an intense source region observed most often near 2200 LMT and 70° invariant latitude. It is directly related to inverted V electron precipitation events with an electron-to-wave energy conversion efficiency of the order of 0.1 to 1%.
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