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Ann. Geophys., 24, 1617-1623, 2006
www.ann-geophys.net/24/1617/2006/
© European Geosciences Union 2006
VHF radar observations of the dip equatorial E-region during sunset in the Brazilian sector
C. M. Denardini, M. A. Abdu, E. R. de Paula, C. M. Wrasse, and J. H. A. Sobral
Instituto Nacional de Pesquisas Espaciais – P.O. Box 515 – S. J. Campos, SP, Brazil
Abstract. Using the RESCO 50 MHz backscatter radar (2.33° S, 44.2° W,
DIP: –0.5), at São Luís, Brazil, we obtained Range Time Intensity (RTI)
maps covering the equatorial electrojet heights during daytime and evening.
These maps revealed a scattering region at an altitude of about 108 km
during the sunset period. The type of 3-m irregularity region we present
here has not been reported before in the literature, to our knowledge. It
was mainly observed around the Southern Hemisphere summer-solstice period,
under quiet magnetic activity condition. The occurrence of this echo region
coincides in local time with the maximum intensity of an evening pre-reversal
eastward electric field of the ionospheric F-region. A tentative explanation
is proposed here in terms of the theory of the divergence of the equatorial
electrojet (EEJ) current in the evening ionosphere presented by Haerendel
and Eccles (1992), to explain the partial contribution of the divergence to the
development of the pre-reversal electric field. The theory predicts an enhanced
zonal electric field and hence a vertical electric field below 300 km as a
consequence of the EEJ divergence in the evening. The experimental results
of the enhanced echoes from the higher heights of the EEJ region seem to provide
evidence that the divergence of the EEJ current can indeed be the driver of
the observed scattering region.
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