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.

We explore the ionospheric effects of prompt penetration electric fields (PPEFs) for a variety of interplanetary magnetic field directions. We use the great magnetic storm of 30–31 October as an example of PPEF effects. For intense southward interplanetary magnetic fields (IMFs), inward plasma sheet convection occurs with the result of magnetospheric ring current formation and an intense magnetic storm. Concurrent with the above, positive phase ionospheric storms occur in the dayside, and negative phase ionospheric storms occur on the nightside, the topics of this paper. The dayside ionospheric storms due to PPEFs are characterized by transport of near‐equatorial plasma to higher altitudes and latitudes, forming a giant plasma fountain. These features are part of what is called the dayside ionospheric superfountain (DIS). For these southward IMFs, dusk and dawn plasma are predicted to be transported toward the dayside. For northward IMFs, negative phase ionospheric storms are expected on the dayside if the PPEFs indeed reach that region of space. IMF By components are expected to have weak or neglible ionospheric effects. On the basis of PPEF arguments, intervals of IMF By should not be related to geomagnetic storms (they are not). IMF By intervals should, however, cause a shearing of the magnetotail, a feature that has been previously reported in the literature.

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,

In the equatorial atmosphere, oscillations with periods of 3 to 4 days have been observed in the meteor radar zonal wind at Cariri (7.4°S, 36.5°W), in the ionospheric minimum virtual height h'F and the maximum critical frequency foF2 at Fortaleza (3.9°S, 38.4°W), and in the TIMED/SABER satellite temperature data in the stratosphere‐mesosphere. Wavelet analyses of these time series reveal that the 3–4‐day oscillation was observed for all of these data during the period from March 1 to 11, 2005. From the characteristics of the downward phase propagation (wavelength of ∼40 km), longitudinal and latitudinal extension, we conclude that this oscillation must be a 3.5–day Ultra Fast Kelvin (UFK) wave. This is the first report of clear evidence of propagation of a UFK wave from the stratosphere to the ionosphere. The UFK wave could have an important role in the day‐to‐day variability of the equatorial ionosphere evening uplift.