F-REGION IONOSPHERIC IRREGULARITIES OVER KING-GEORGE-ISLAND AND ARGENTINE ISLANDS - A COMPARATIVE-STUDY

Spread-F is caused by the presence of ionospheric electron concentrati on irregularities of scale-size of order 5 lan at F-region altitudes. Estimates of spread-F in the vicinity of the maximum plasma frequency of the F layer (foF2) have been determined at 15 min intervals from io nograms recorded over a ten day period (1-10 May 1986) both at Marsh ( 62.2-degrees-S,58.9-degrees-W), King George Island, and Faraday (65.2- degrees-S,64.3-degrees-W), Argentine Islands. The interval, at low sol ar activity, includes periods of quiet and disturbed geomagnetic activ ity. Spread-F is observed on every night at both stations. It is more frequent, slightly more intense and starts earlier at Argentine Island s than at King George Island. On most nights, spread-F ceases about lo cal sunrise at 120 km altitude at both stations. On the days of highes t geomagnetic activity, the onset of spread-F is delayed compared with days of lower activity. Spread-F is usually most intense on the night (s) following largest geomagnetic activity level, as measured by the g eomagnetic index, Kp. The growth rate of the plasma instability proces ses causing the ionospheric irregularities is inversely related to ele ctron concentration (foF2(2)), amongst other parameters. Thus the lowe r foF2 values over Argentine Islands are consistent with more spread-F being observed by the higher latitude observatory. However, no firm r elationship between the absolute value of foF2, the horizontal gradien t of foF2 between the two observatories, and the onset of spread-F, is found. Thus it has not been possible to determine uniquely the instab ility process responsible for the formation of the plasma irregulariti es.