Ionospheric loop currents and associated ULF oscillations at geosynchronous altitudes during preonset intervals of substorm aurora

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AbstractA substorm aurora was observed at 04:52 UT on 27 January 1986 by an all‐sky imager installed at Shamattawa (66.3°N, 336.0° in corrected geomagnetic coordinates) and a magnetometer on board a conjugate satellite (GOES 6) at geosynchronous altitudes. In the preonset intervals lasting for approximately 50 min prior to the expansion onset, an equatorward drift of the auroras beginning from 71°N to 64°N was observed. Meanwhile, GOES 6 observed ULF oscillations in 25–200 s periods at geosynchronous altitudes after about 10 min following the start of the equatorward drift. During the equatorward drift of the arc, the flow reversal occurred, where the auroral arc propagating eastward was replaced by a westward propagation. Simultaneously, the major axes of the ULF oscillations rotated clockwise by ~90° in the equatorial plane. We conclude that the ULF oscillations are azimuthally small‐scale Alfvén waves excited in the magnetosphere by field‐aligned currents associated with ionospheric loop currents.