The electric fields in the ionospheric E and F regions near the magnetic equator often show a strong eastward enhancement shortly after sunset and before the eastward (normally) daytime field reverses to westward. Several theoretical models of the low-latitude fields suggest that this enhancement is caused mainly or entirely by F region winds (the F region dynamo), but some authors have suggested that it could be produced solely by E region tidal winds. We give here additional calculations and arguments in support of the F region source. The enhancement of the eastward field for an eastward F region wind tums out to be a simple direct consequence of the fact that after sunset the ionospheric conductivity decreases far more rapidly in the E region than in the F region. 13,72313,724 FARLEY ET AL.' BRIEF REPORT

Incoherent scatter radar measurements at ß Jicamarca are used to study the effects of large solar fluxes and magnetic activity on the F region vertical plasma drifts. The average drifts from the two last solar maxima are almost identical except in the late afternoon-early evening sector where their variations with solar flux and magnetic activity are strongly season dependent. The average evening winter (May-August) drifts appear to remain almost constant after a certain solar flux level is reached but increase with magnetic activity. The equinoctial evening drifts increase systematically with solar-flux but decrease with magnetic activity. Very large prereversal enhancement velocities, up to about 80 m/s, were often observed during the 1978-1981 equinoctial periods when the solar flux was very high. Comparison of incoherent scatter radar drifts with vertical velocities inferred from ionosonde observations indicate that the latter technique substantially underestimates the plasma drifts during periods of large solar fluxes except during winter.

By a sample record we show that the radio beacon experiments which utilize the Faraday rotation technique and the scintillation observations can be used to observe equatorial ionization bubbles. In a two‐hour period 5 isolated bubbles have been identified. The depleted total electron content for one such bubble is 2.2×1016 electrons/m² and the east‐west dimension is about 72 km. This translates to a total depletion of 1.6×1021 electron per meter in the north‐south direction.

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[1] Amplitude scintillations in satellite signals can cause errors in communications, because of signal fading, but can be very useful for scientists trying to improve their understanding of the physics of the ionosphere. Usually, magnetic storms are expected to affect the ionosphere in such way as to increase ionospheric irregularities responsible for scintillations. To help change the view of scientists and engineers, in this respect, we show that amplitude scintillation on GPS signals show dramatic decrease during selected magnetic storms, at Brazilian GPS stations. These stations are located on magnetic latitudes that go from equatorial (Sã o Luís) to low-latitude (Sã o José dos Campos and Cachoeira Paulista) so that a region of several thousand kilometers is represented by the data. We present 4 months of data chosen from 2003 to 2005 to represent the strongest storms during each scintillation season. Although there is lack of data for some days from the different stations, it is possible to see, especially for the Halloween Storm (October 2003), that scintillations are attenuated in this wide range of latitudes. During magnetically calm periods scintillations are strong, in this region, from August to March, during solar maxima. Although the data are clear about the attenuation of scintillations during greater magnetic storms, it is not possible to easily conclude which physical mechanism was responsible for this phenomenon, even with the aid of more detailed data like Dst and AE.