EQUATORIAL IONOSPHERE SUNSET ELECTRODYNAMICS IN THE AMERICAN SECTOR FROM SUNDIAL DECEMBER 1988 CAMPAIGN RESULTS

During the 3-4 December 1988 SUNDIAL campaign, coordinated ionospheric measurements were conducted using the low latitude networks of ionoso ndes and magnetometers distributed at different longitude sectors of t he globe and the Jicamarca radar. The results for the American longitu de sector are analysed here to examine outstanding features of the sun set F-region electric field developments at Jicamarca and at the Brazi lian stations, Fortaleza and Cachoeira Paulista, during the 8-day inte rval that represented predominantly quiet (except 3 and 11 December th at were weakly disturbed) geomagnetic conditions. A comparative study, carried out for the first time, on the simultaneous measurements of F -region plasma vertical drift velocity by Jicamarca radar and the equi valent drift velocity measured by Fortaleza ionosonde testifies the ex istence, previously known, of the large difference in the evening prer eversal electric field developments at the two locations. Further comp arison of simultaneous vertical drifts over Fortaleza and over Cachoei ra Paulista points to the existence of a height gradient in the zonal evening electric field for the near-solar maximum condition of this ca mpaign measurements. A physical explanation for these effects is attem pted using a simplified model of the equatorial evening electric field development processes that considers also asymmetrie conjugate E laye r sunset decay conditions.