DIURNAL AND SEASONAL VARIABILITY OF THE SOUTHERN-HEMISPHERE MAIN IONOSPHERIC TROUGH FROM DIFFERENTIAL-PHASE MEASUREMENTS

A differential-phase technique utilizing the radio transmissions of NN SS satellites was used to make measurements of the latitudinal variati ons of ionospheric vertical total electron content (TEC) in the southe rn mid-latitude trough region from the location of Macquarie Island (a cis-auroral site; geographic coordinates 54.5-degrees-S, 154.95-degre es-E, geomagnetic coordinates 64.5-degrees-S, 177.67-degrees-E, L = 5. 38) for a period of 14 months during 1987-1989. The differential-phase method provided a means of observing a relatively large expanse of io nosphere whilst centered on the cis-auroral region which was well suit ed for trough studies. By monitoring the two transmitted radio signals at 150 and 400 MHz from the Navy Navigation Satellite System (NNSS) p olar orbiting satellites it was possible to deduce the latitudinal var iation of ionospheric vertical TEC for the duration of the satellite p ass. The absolute TEC was derived from Faraday-rotation and ionosonde data obtained during the same period. The main findings of this work h ave been the high incidence of daytime troughs for all seasons and the relative low incidence of night-time troughs. Both summer and vernal equinox seasons display a greater occurrence frequency of daytime trou ghs than the winter and autumnal equinox seasons. Winter-time troughs at any time are less frequent than for any other season. Comparisons w ith the northern-hemisphere trough display a marked difference in occu rrence frequency and distribution of troughs. An attempt to explain so me of these features in the light of the high-latitude convection theo ry is offered. Case studies are given for all seasons to highlight the se findings.