Abstract.A physical mechanism and the location and latitudinal extent of an additional layer, called the F3 layer, that exists in the equatorial ionosphere are presented. A statistical analysis of the occurrence of the layer recorded at the equatorial station Fortaleza (4øS, 38øW; dip 9øS) in Brazil is also presented. The F3 layer forms during the morning-noon period in that equatorial region where the combined effect of the upward ExB drift and neutral wind provides a vertically upward plasma drift velocity at altitudes near and above the F2 peak. This velocity causes the F2 peak to drift upward and form the Fa layer while the normal F• layer develops at lower altitudes through the usual photochemical and dynamical effects of the equatorial region. The peak electron density of the Fa layer can exceed that of the Fu layer. The F3 layer is predicted to be distinct on the summer side of the geomagnetic equator during periods of low solar activity and to become less distinct as the solar activity increases. Ionograms recorded at Fortaleza in 1995 show the existence of an Fa layer on 49% of the days, with the occurrence being most frequent (75%) and distinct in summer, as expected. During summer the layer occurs earlier and lasts longer compared to the other seasons; on the average, the layer occurs at around 0930 LT and lasts for about 3 hours. The altitude of the layer is also high in summer, with the mean peak virtual height being about 570 kin. However, the critical frequency of the layer (foF3) exceeds that of the Fu layer (foF•) by the largest amounts in winter and equinox; foFa exceeds foF2 by a yearly average of about 1.3 MHz.

[1] Equatorial ionospheric plasma bubble irregularity development and dynamics during the major magnetospheric storm of 26 August 1998 are investigated using the data collected by a multistation and multi-instrument diagnostic network operated at equatorial and low latitude sites in Brazil, and auroral electrojet activity (AU/AL), IMF, and D st indices. A magnetospheric disturbance onset in the morning of 26 August 1998 was initiated by a solar wind shock and associated IMF Bz polarity reversals and ssc that were soon followed by a succession of substorm-like auroral electrojet (AE) intensifications and D st development. An IMF Bz southward turning and associated AE intensifications in the Brazilian dusk sector produced intense prompt penetration eastward electric field that caused large F region vertical drift and consequently the developments of intense postsunset equatorial anomaly and a series of intense plasma bubbles, the latter event lasting the entire night, as observed by digital ionosondes at São Luís (2.33°S, 315.8°E, dip angle: À.5°) and Fortaleza (3.9°S, 321.55°W, dip angle: À9°) and an all-sky imager, two scanning photometers, and a Digisonde at the lowlatitude site Cachoeira Paulista (22.6°S, 315°E; dip angle: À28°). A notable aspect of the dynamics of the bubbles was their initially very low eastward drift velocity which turned into steadily increasing westward velocity that lasted till early morning hours. The results show for the first time a relationship between the zonal drift velocities of optically observed large-scale bubbles (tens to hundreds of kilometers) and that of the smaller scale (kilometer sizes) structures as observed by a digital ionosonde. The results point to the dominant role of a disturbance dynamo associated westward thermospheric wind to maintain the plasma irregularity drift increasingly westward going into postmidnight hours. As an important finding, the results further show that significant contribution to the westward plasma bubble irregularity drift, normally attributed to disturbance dynamo effect, could arise from prompt penetration disturbance zonal electric field, in the course of a disturbance sequence lasting several hours. Such effect is attributed to Hall electric field arising from the primary disturbance zonal electric field, under enhanced nighttime ionospheric conductivities produced possibly by storm associated particle precipitation, in the Brazilian longitude sector in agreement with recent evidences [Abdu et al., 1998b].

Abstract. The equatorial plasma fountain and equatorial anomaly in the ionospheres over Jicamarca (77øW), Trivandrum (77øE), and Fortaleza (38øW) are presented using the Sheffield University plasmasphere-ionosphere model under magnetically quiet equinoctial conditions at high solar activity. The daytime plasma fountain and its effects in the regions outside the fountain lead to the formation of an additional layer, the F 3 layer, at latitudes within about plus or minus 10 ø of the magnetic equator in each ionosphere. The maximum plasma concentration of the F 3 layer, which occurs at about 550 km altitude, becomes greater than that of the F2 layer for a short period of time before noon when the vertical E x B drift is large. Within the F 3 layer the plasma temperature decreases by as much as 100 K. The ionograms recorded at Fortaleza on January 15, 1995, provide observational evidence for the development and decay of an F 3 layer before noon. The neutral wind, which causes large north-south asymmetries in the plasma fountain in each ionosphere during both daytime and nighttime, becomes least effective during the prereversal strengthening of the upward drift. During this time the plasma fountain is symmetrical with respect to the magnetic equator and rises to over 1200 km altitude at the equator, with accompanying plasma density depletions in the bottomside of the underlying F region. The north-south asymmetries of the equatorial plasma fountain and equatorial anomaly are more strongly dependent upon the displacement of the geomagnetic and geographic equators (Jicamarca and Trivandrum) than on the magnetic declination angle (Fortaleza). IntroductionThe horizontal orientation of the geomagnetic field at the geomagnetic equator is known to be the basic reason for the active nature of the low-latitude ionosphere, which is characterized by the equatorial electrojet, equatorial plasma fountain, equatorial anomaly, plasma bubbles, and spread F. The equatorial plasma fountain and equatorial anomaly arise from the vertical upward drift of plasma across the geomagnetic field lines at equatorial latitudes due to E

[1] An investigation of the evening prereversal enhancement in the equatorial zonal electric field (PRE) based on ionosonde data show that the PRE development process is coupled with the sporadic E layer formation in the evening over Fortaleza. Larger PRE amplitudes are associated with disruption of the Es layer, whereas for smaller PRE amplitudes such disruption does not occur, in general. The Es layer disruption does not occur also when the PRE amplitude decreases or is inhibited under a disturbance dynamo electric field. The disruption of these layers is followed by their reconstitution after a break of $3 hours. An examination of the relative role of the electric field and winds on ion velocity convergence process shows that the Es layer formation from a shearing (or height-independent and westward) zonal wind is directly influenced by a vertical electric field (but not by zonal electric field). Measurements of the Es patch zonal drift velocities by a digital ionosonde seem to support the role of a westward wind in the Es layer formation. The observed association between the PRE and Es layer disruption/formation is shown to arise from sunset-related vertical electric field development originating from the E and F region electrodynamic coupling processes. The results demonstrate the competing influences of the vertical electric field and the zonal wind in the evening Es layer processes. Since the PRE is responsible for the equatorial spread F (ESF) development, its relationship with the Es layer is discussed in the context of the day-to-day variability of the ESF.INDEX TERMS: 2415 Ionosphere: Equatorial ionosphere; 2411 Ionosphere: Electric fields (2712); 2437 Ionosphere: Ionospheric dynamics; 2427 Ionosphere: Ionosphere/atmosphere interactions (0335); 2435 Ionosphere: Ionospheric disturbances; KEYWORDS: equatorial ionosphere, equatorial prereversal electric field, sporadic E-layer, E-layer winds, magnetic disturbances, E-F-region electrical coupling Citation: Abdu, M. A., J. W. MacDougall, I. S. Batista, J. H. A. Sobral, and P. T. Jayachandran, Equatorial evening prereversal electric field enhancement and sporadic E layer disruption: A manifestation of E and F region coupling,

[1] Polar cap ionospheric measurements are important for the complete understanding of the various processes in the solar wind-magnetosphere-ionosphere system as well as for space weather applications. Currently, the polar cap region is lacking high temporal and spatial resolution ionospheric measurements because of the orbit limitations of space-based measurements and the sparse network providing ground-based measurements. Canada has a unique advantage in remedying this shortcoming because it has the most accessible landmass in the high Arctic regions, and the Canadian High Arctic Ionospheric Network (CHAIN) is designed to take advantage of Canadian geographic vantage points for a better understanding of the Sun-Earth system. CHAIN is a distributed array of ground-based radio instruments in the Canadian high Arctic. The instrument components of CHAIN are 10 high data rate Global Positioning System ionospheric scintillation and total electron content monitors and six Canadian Advanced Digital Ionosondes. Most of these instruments have been sited within the polar cap region except for two GPS reference stations at lower latitudes. This paper briefly overviews the scientific capabilities, instrument components, and deployment status of CHAIN. This paper also reports a GPS signal scintillation episode associated with a magnetospheric impulse event. More details of the CHAIN project and data can be found at http:// chain.physics.unb.ca/chain.