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[1] Analysis of the MLT region winds measured by a meteor radar and the evening F region vertical plasma drift (prereversal zonal electric field -PRE) measured by digisondes over low latitude sites in Brazil, provide evidence of planetary wave (PW) scale oscillations of episodic nature simultaneously at mesospheric and F region heights. $4-day and 7-day periods are found to dominate the event analyzed. The PW scale oscillations in the PRE produces strong modulation in the equatorial spread F (ESF) irregularity processes as diagnosed by the digisondes. Considerations on the PRE development mechanism involving the E layer integrated conductivity including the effect of metallic ions and tidal winds point to the source of the PRE oscillations to be PW modulation of E region tidal winds. The PW oscillations in PRE appear to be an important source of the day-to-day variability in the ESF.

In the present work, we analyze the competition between tidal winds and electric fields in the formation of blanketing sporadic E layers (Es b ) over São Luís, Brazil (2° 31′ S, 44° 16′ W), a quasi-equatorial station. To investigate this competition, we have used an ionospheric E region model (MIRE) that is able to model the Es b layers taking into account the E region winds and electric fields. The model calculates the densities for the main molecular and metallic ions by solving the continuity and momentum equations for each of the species. Thus, the main purpose of this analysis is to verify the electric fields role in the occurrence or disruption of Es b layers through simulations. The first results of the simulations show that the Es b layer is usually present when only the tidal winds were considered. In addition, when the zonal component of the electric field is introduced in the simulation, the Es b layers do not show significant changes. However, the simulations show the disruption of the Es b layers when the vertical electric field is included. In this study, we present two specific cases in which Es b layers appear during some hours over São Luís. We can see that these layers appear when the vertical electric field was weak, which means that the tidal components were more effective during these hours. Therefore, the vertical component of the electric field is the main agent responsible for the Es b layer disruption. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

[1] The role of the molecular and metallic ions in the formation of sporadic E layers (Es layers) has been an active area of research for many years. Ionosonde data of the Es layer parameter, fbEs (the blanketing frequency of sporadic layer), and ftEs (the top frequency of Es trace), obtained over the Brazilian sites, Fortaleza, and São Luis, show that the diurnal variations of these parameters often present a disruption that is coincident with the evening prereversal electric field/vertical drift enhancement, to reappear after about 2 hours. In this work we have developed a model of the E region with the objective to describe all the phases of the life cycle of the sporadic layer and its relation to the evening vertical electric field enhancement that is associated with the development of the prereversal zonal electric field enhancement. The model calculates the densities of NO + , O 2 + , O + , N 2 + , and Fe + in the height region 86-140 km by simultaneously solving the continuity and momentum equations for each of the species. For low latitudes this model shows that winds and electric fields each play distinct roles in the vertical transport of plasma leading to Es layer formation. The results of our numerical simulation confirm the dominant role of the vertical electric field as the cause of the inhibition/disruption of the sporadic layer in the evening. More specifically, the results reveal that an upward directed (positive) vertical electric field is capable of disrupting an ongoing sporadic layer, whereas a downward (negative) electric field favors its formation or even intensification, thus corroborating the conclusions of the recent study by Abdu et al. (2003).Citation: Carrasco, A. J., I. S. Batista, and M. A. Abdu (2007), Simulation of the sporadic E layer response to prereversal associated evening vertical electric field enhancement near dip equator,