ONSET CONDITIONS OF EQUATORIAL (RANGE) SPREAD-F AT FORTALEZA, BRAZIL,DURING THE JUNE SOLSTICE

Case studies are made of the ionospheric, thermospheric, and geomagnet ic conditions associated with the occasional onset of range spread F a t Fortaleza (4 degrees S, 38 degrees W, dip latitude -1.8 degrees, and magnetic declination 21 degrees W), Brazil, in the June solstice, a s eason in which frequency spread F is typically seen in the postsunset hours (after 2000 LT). It is found that the F layer experiences quite consistently a large vertical drift early in the evening hours (1815-1 915 LT) on days of range spread F in comparison to days of only freque ncy spread F. The anomalously large dusk time vertical plasma drift oc curs under both geomagnetically disturbed and quiet conditions. There is no significant change in the pattern of meridional neutral winds (a t and prior to the time of onset of range spread F) estimated from the data of F layer peak height (h(MAX)) at the low-latitude station, Cac hoeira Paulista (23 degrees S, 45 degrees W, and dip 26.5 degrees S), using a modified form of the servo model. The results strongly suggest that though the meridional (poleward) wind is very effective in inhib iting the growth of Rayleigh-Taylor (R-T) instability and hence range spread F at Foltaleza during the June solstice as shown by Maruyama [1 996], its variability does not play an important role in creating favo rable conditions for R-T instability on a day-to-day basis. The prereq uisite for the occasional occurrence of range spread F at Fortaleza in the June solstice seems to be the presence of an impulsive and large F layer vertical plasma drift, a condition favorable for destabilizing the bottomside F region through R-T instability mechanism because of the high altitude of the layer.