Instruments on the Auroral Turbulence rocket detected several intervals of weak electromagnetic oscillations at frequencies of 6-13 Hz in a strongly flickering auroral arc. These oscillations have amplitudes of up to 5B • 3 nT and 5E • 4 mV/m and have downward field-aligned Poynting fluxes of up to • 10 -s W/m 2. Fluctuations in the parallel electron flux at about 9 Hz were observed in association with the strongest of these oscillations. Simultaneous groundbased optical data show that the arc was flickering at frequencies of 8-15 Hz. The observed frequencies would match the oxygen cyclotron frequency at • 4500 km altitude. In one wave/particle event the apparent lag of the waves behind the modulated electrons implies a modulation source altitude of 2500-5000 km. We interpret these waves as electromagnetic ion cyclotron waves originating in the auroral acceleration region. Paper number 95GL02409 0094-8534/95/95GL-02409503.00 campaign of 1984 [Ternerin e! al., 1986]. In the model of Ternerin et al. [1986, 1993], oblique electromagnetic oxygen and hydrogen cyclotron waves accelerate the electrons which cause the flickering aurora. Many satellite observations have been made of electromagnetic oxygen cyclotron waves, most recently with edge of auroral arcs, Eos Trans. A GU, 75, 572, 1994.

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Abstract. The auroral absorption spike event, occurring generally at the onset of a substorm, is distinguished by its high intensity and short duration. This paper reports the presence of a fine structure within the spike event. Analysis of selected examples using the Morlet wavelet shows that within the 1-2 min duration of the spike are significant modulations with periodicities in the bands 15-60 s (67-16 mHz), and 5-•0 s (200-•00 mHz), the former being the stronger. The slower fluctuations can amount to more than 10% of the absorption, and they were observed in every example (seven out of nine) in which the spike was moving poleward. They were absent in the other two cases, when the spike moved equatorward. In the examples studied, the 15-60 s absorption pulsations were accompanied by magnetic micropulsations of impulsive type (Pi) having a periodity that was similar or harmonically related. The connection is only close while the spike event is moving. Consideration of the details suggests that both the magnetic and the absorption pulsations are related to the acceleration process at substorm onset, the flt•x of energetic particles into the auroral zone producing the radio absorption being modulated with, though not by, the geomagnetic field variations. The 5-10 s pulsations, which are considerably weaker, appeared in both the absorption and the magnetic records, but in this case with no obvious connection between them.