SATELLITE-OBSERVATIONS OF ZONAL ELECTRIC-FIELDS NEAR SUNRISE IN THE EQUATORIAL IONOSPHERE

We report here on a number of examples of anomalous enhancements of ea stward electric fields near sunrise in the equatorial ionospheric F-re gion. These examples were selected from the data base of the equatoria l satellite, San Marco D (1988), which measured ionospheric electric f ields during a period of solar minimum. The eastward electric fields r eported correspond to vertical plasma drifts. The examples studied her e are similar in signature and polarity to the pre-reversal electric f ield enhancements seen near sunset from ground-based radar systems. Th e morphology of these sunrise events, which are observed on about 14% of the morning-side satellite passes, are studied as a function of loc al zonal velocity, magnetic activity, geographic longitude and altitud e. The nine events studied occur at locations where the zonal plasma f low is generally measured to be eastward, but reducing as a function o f local time and at satellite longitudes where the magnetic declinatio n has the opposite polarity as the declination of the sunrise terminat or.