Observations of MSTIDs/GWs at the F2 layer heights in the near equatorial region.

Ionospheric vertical sounding observations, using a digital ionosonde (Canadian Advanced Digital Ionosonde(CADI)) , arebeing carried out on a routine basis at Manaus (2,9 0 S; 60,0 0 W; dip latitude 6,4 N, hereafter referred to as MAN), Brazil, locatedbetween the geographic and geomagnetic dip equators, since August 2002, (Fig. 1). The medium scale traveling ionosphericdisturbance (MSTID) signatures, induced by gravity waves (GWs), in the F2 layer can be observed sometimes during daytime in theiso-frequency plots of virtual height daily variations for six fixed frequencies (3, 4, 5, 6, 7, and 8 MHz) which show quasi-periodicoscillations (crests and valleys). The crests and valleys when seen in close frequencies present a phase difference (i.e., first it isobserved at higher frequency then at lower frequency), indicating a downward phase velocity (Fig. 2C). However, these MSTIDscannot be observed during the pre-reversal period because of the presence of strong eastward electric field in the equatorialregion which possibly swamps the MSTID signatures (Fig. 2). This paper presents and discusses the relationship betweenpropagation of MSTID/GW at F-region heights and generation of equatorial spread F (ESF) (Fig. 2A). Also, a comparison betweenESF observed in the equatorial region of the Brazilian sector and other sectors will be presented.