Arrival of Ionospheric Troughs and Oblique Spread-F in the Wake of Large-Scale Travelling Ionospheric Disturbances During the Intense Auroral Storms of August 22 and 23, 1990

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A modem digital ionosonde (Digisonde 256) recording the amplitude, angle-of-arrival, Doppler shift, and polarisation of MF and HF (1-10 MHz) echoes from the ionosphere was operated at the mid-latitude station of Beveridge (geographic 37.5°S, 144.9°E), located 40 km north of Melbourne, Australia, during the intense auroral storms of August 22 and 23, 1990. We changed the digisonde antenna design to permit the alternate recording of near-vertical angle-of-arrival ionograms, and southward-looking Doppler ionograms sensitive to oblique echoes at great ranges. This experimental design permitted us to track the equatorward migration of radio aurora associated with the poleward wall of the mid-latitude trough, the small-scale “slant-F” irregularities and medium-scale travelling ionospheric disturbances (TIDs) detected within the trough, and distinctive oblique spread-F patches located in proximity to the equatorward edge of the auroral zone; all arriving in response to large auroral electrojet activity. These observations graphically illustrate the processes by which the magnetosphere dumps energy into the high-latitude ionosphere when the geomagnetic index is high (ΣK_p = 46 on August 23), leading to the generation of large-scale TIDs which precede the arrival of, and possibly contribute to the production of, the aforementioned menagerie of high mid-latitude irregularities. We illustrate the five forms (broadly speaking) of spread-F observed and offer plausible explanations for their cause. We corroborate our interpretation of oblique ionogram measurements by using DMSP/F8 and F9 precipitating particles satellite data, magnetometer measurements at Canberra (Δ = 45.9°S) (geomagnetic latitudes hereafter) and at Macquarie Is. (64.4°S) in the auroral zone, and routine ionograms (i.e., limited sensitivity to oblique echoes, and no angle-of-arrival or Doppler shift information) recorded at other mid-latitude stations. For example, during the night of August 23, radio aurora were observed to move equatorwards towards Beveridge (48.0°S) where a severe spread-F event occurred due to TIDs generated in the region defined by Macquarie Island north to Hobart (53.7°) (i.e., the source location probably changed in latitude as the aurora moved equatorward). The TIDs observed at Beveridge were also observed at Canberra (where mild spread-F was observed), and as far north as Norfolk Island (36.0°S) where they had little impact on the ionosphere.