Anomalous auroral electron distributions due to an artificial ion beam in the ionosphere

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Abstract
We report the results of an active experiment in which the auroral ionosphere was perturbed by the operation of an ion gun, which injected approximately 100 mA of 25 eV Ar+ ions at upgoing pitch angles over a discrete auroral arc. The gun was operated during five periods on the downleg of the flight as the payload descended from 220 km to 120 km altitude along the local magnetic field lines. A brief (< 100 ms) negative payload potential excursion was observed at the onset of each gun operation, followed by quasi‐steady conditions for the remainder of each period. The major steady effects were (1) the excitation of intense broad band electric field fluctuations, particularly at low frequencies 0‐10 kHz, and (2) the appearance of streaming and isotropic heating in different parts of superthermal electron velocity space. We have explored a scenario that appeals to such a wave spectrum as an obstacle to auroral parallel current and predicts this type of electron behavior. The waves are viewed as traps for the thermal electrons and scatterers of the faster electrons. In this scenario, electron runaway or streaming is expected between the trapping speed and the critical velocity for cyclotron interactions with the waves. These streaming electrons carry the current that would be carried by thermals or energetic electrons in the absence of the waves. Isotropic heating is expected near the critical speed where Doppler resonance occurs and is found near 12000 km/s (∼50 υte, where υte is the electron thermal speed). It may be inferred that waves are excited primarily at wavelengths of several meters and longer. A current of ∼1 µA m−2 is carried by the streaming electrons. Energetic considerations suggest that the heated electrons derive their energy from an external source rather than from the ion beam itself; the auroral generator appears to be more than adequate. The gun‐associated electrons were anomalous in the sense that their anisotropy was the opposite of that observed in the natural aurora, where electrons are isotropic at low energy and often field‐aligned at higher energy.