ARRIVAL OF IONOSPHERIC TROUGHS AND OBLIQUE SPREAD-F IN THE WAKE OF LARGE-SCALE TRAVELING IONOSPHERIC DISTURBANCES DURING THE INTENSE AURORAL STORMS OF AUGUST 22 AND 23, 1990

A modern digital ionosonde (Digisonde 256) recording the amplitude, an gle-of-arrival, Doppler shift, and polarisation of MF and HF (1-10 MHz ) echoes from the ionosphere was operated at the midlatitude station o f Beveridge (geographic 37.5 degrees S, 144.9 degrees E), located 40 k m north of Melbourne, Australia, during the intense auroral storms of August 22 and 23, 1990. We changed the digisonde antenna design to per mit the alternate recording of near-vertical angle-of-arrival ionogram s, and southward-looking Doppler ionograms sensitive to oblique echoes at great ranges. This experimental design permitted us to track the e quatorward migration of radio aurora associated with the poleward wall of the mid-latitude trough, the small-scale ''slant-F'' irregularitie s and medium-scale travelling ionospheric disturbances (TIDs) detected within the trough, and distinctive oblique spread-F patches located i n proximity to the equatorward edge of the auroral zone; all arriving in response to large auroral electrojet activity. These observations g raphically illustrate the processes by which the magnetosphere dumps e nergy into the highlatitude ionosphere when the geomagnetic index is h igh (Sigma K-p = 46 on August 23), leading to the generation of large- scale TIDs which precede the arrival of, and possibly contribute to th e production of, the aforementioned menagerie of high mid-latitude irr egularities. We illustrate the five forms (broadly speaking) of spread -F observed and offer plausible explanations for their cause. We corro borate our interpretation of oblique ionogram measurements by using DM SP/F8 and F9 precipitating particles satellite data, magnetometer meas urements at Canberra (Lambda = 45.9 degrees S) (geomagnetic latitudes hereafter) and at Macquarie Is. (64.4 degrees S) in the auroral zone, and routine ionograms (i.e., limited sensitivity to oblique echoes, an d no angle-of-arrival or Doppler shift information) recorded at other mid-latitude stations. For example, during the night of August 23, rad io aurora were observed to move equatorwards towards Beveridge (48.0 d egrees S) where a severe spread-F event occurred due to TIDs generated in the region defined by Macquarie Island north to Hobart (53.7 degre es) (i.e., the source location probably changed in latitude as the aur ora moved equatorward). The TIDs observed at Beveridge were also obser ved at Canberra (where mild spread-F was observed), and as far north a s Norfolk Island (36.0 degrees S) where they had little impact on the ionosphere.