The importance of diffusion, perpendicular electrodynamic drift, and neutral wind on the generation and modulation of the equatorial plasma fountain of the Earth's ionosphere is studied using the Sheffield University plasmasphere-ionosphere model for the ionosphere above Jicamarca under magnetically quiet equinoctial conditions at medium solar activity. The effects of the fountain, which include the equatorial anomaly, are also investigated. As expected, the F region electrodynamic (E x B) drift generates the plasma fountain and the anomaly, which are symmetric with respect to the magnetic equator. The neutral wind introduces asymmetries, with larger plasma flow (toward the hemisphere of stronger poleward wind) and stronger anomaly crest occurring in opposite hemispheres. During daytime, when the drift is upward, the fountain rises to about 800 km altitude at the equator and covers about +300 magnetic latitude; outside the reach of the fountain, plasma flows toward the equator from both hemispheres. This convergence of plasma leads to the formation of an additional layer (called the G layer) within 4-100 of the magnetic equator during the prenoon hours when the drift is large. At the magnetic equator, the maximum plasma concentration of the G layer can be greater than that of the F layer for a short period of time just before noon and when the drift starts to decrease. In the evening, soon after the drift turns downward, the fountain becomes a reverse fountain with supply of ionization from both hemispheres from regions outside the fountain. The reverse fountain acts as the main source for the nighttime increase in ionization at equatorial anomaly latitudes, with some contribution from the prereversal strengthening of the forward fountain. The importance of the prereversal strengthening of the forward fountain, which rises to about 1000 km altitude at the equator, and the following reverse fountain on the generation and propagation of plasma bubbles and spread F irregularities is discussed. It is also shown that the equatorial anomaly in the vertical ionospheric electron content need not be as pronounced as the interpretation of the observations suggests. Paper number 95JA01555. 0148-0227/95/95JA-015555 05.00 ized by a trough at the equator and crests at about 4-170 magnetic latitude [Appleton, 1946]; the crest-totrough ratio (about 1.5 in daytime peak electron density, N[nax) and the position of the crests vary with various geophysical conditions. Many theories, like the diffusion theory [Mitra, 1946] and the electrodynamic drift theory [Martyn, 1955], have been put forward to explain the anomaly. The diffusion theory has been shown to be important but not sufficient to explain the observations [Rishbelh et al., 1963]. The electrodynamic drift theory, on the other hand, has been successful in explaining the observations [Bramley and Pearl, 1965; Moffett and Hanson, 1965]. According to the drift theory, the north-south geomagnetic field combined with the daytime east--west ionospheric electric field (both being p...

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] During magnetic storms the ionospheric total electron content (TEC) at low-and midlatitudes often shows great enhancements, which may be associated with mechanisms producing midlatitude storm-enhanced density (SED). The TEC enhancements may result from different ionospheric drivers such as electric fields, neutral winds, and neutral composition effects. To study the importance of the ionospheric drivers in producing the TEC enhancement, we perform numerical simulations for the 29-30 October 2003 superstorm period in the American longitude sector ($ À70°W) using the Sheffield University Plasmasphere Ionosphere Model (SUPIM) with values for the neutral wind, temperature, and composition provided by the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere General Circulation Model (TIEGCM). Various numerical experiments were run to identify the relative importance of the storm-time ionospheric drivers. For carrying out the storm-time SUPIM simulation, the storm-time upward/poleward E Â B drifts are derived from ROCSAT-1 satellite measurements at low and equatorial latitudes and input to SUPIM, while the storm-time neutral wind and composition disturbances are obtained from TIEGCM run. The simulation results presented in this paper, mainly during the evening period, show that the enhanced upward E Â B drifts due to storm-time eastward penetration electric field can expand the low-latitude equatorial ionization anomaly (EIA) to higher latitudes and produce the TEC enhancement. However, by the effect of penetration electric fields alone, the TEC enhancement is less than by combining the storm-generated equatorward neutral winds and the penetration electric fields. Disturbance neutral composition effects decrease the plasma density at higher latitudes and increase it at low and equatorial latitudes. However, the composition effects do not produce a density increase as large as that produced by the neutral-wind and electric-field effects. Our simulations suggest that the storm-generated equatorward neutral winds play an important role in producing the TEC enhancement at low-and midlatitudes, in addition to the eastward penetration electric field.

A physical mechanism of the positive ionospheric storms at low latitudes and midlatitudes is presented through multi‐instrument observations, theoretical modeling, and basic principles. According to the mechanism, an equatorward neutral wind is required to produce positive ionospheric storms. The mechanical effects of the wind (1) reduce (or stop) the downward diffusion of plasma along the geomagnetic field lines, (2) raise the ionosphere to high altitudes of reduced chemical loss, and hence (3) accumulate the plasma at altitudes near and above the ionospheric peak centered at around ±30° magnetic latitudes. Daytime eastward prompt penetration electric field (PPEF), if it occurs, also shifts the equatorial ionization anomaly crests to higher than normal latitudes, up to approximately ±30° latitudes. The positive ionospheric storms are most likely in the longitudes where the onset of the geomagnetic storms falls in the ionization production dominated morning‐noon local time sector when the plasma accumulation due to the mechanical effects of the wind largely exceeds the plasma loss due to the chemical effect of the wind. The mechanism agrees with the multi‐instrument observations made during the supergeomagnetic storm of 7–8 November 2004, with 18 h long initial phase (IP) and 10 h long main phase (MP). The observations, which are mainly in the Japanese‐Australian longitudes where the MP onset was in the morning (0600 LT, 2100 UT), show (1) strong positive ionospheric storms (in Ne, Nmax, hmax, Global Positioning System–total electron content (GPS‐TEC), and 630 nm airglow intensity) in both Northern and Southern hemispheres started at the morning (0600 LT) MP onset and lasted for a day, (2) repeated occurrence of strong eastward PPEF events penetrated after the MP onset and superposed with westward electric field started before the MP onset, and (3) storm time equatorward neutral winds (inferred from 1 and 2). Repeated occurrence of an unusually strong F3 layer with large density depletions around the equator was also observed during the morning‐noon MP.

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] Relative importance of diffusion, electric field, and neutral wind on equatorial plasma fountain and equatorial ionization anomaly (EIA) during a strong daytime eastward prompt penetration electric field (PPEF) event are evaluated using the Sheffield University Plasmasphere Ionosphere Model and the recorded PPEF during the super geomagnetic storm of 9 November 2004. The fountain rapidly develops into a super fountain during the PPEF event. The super fountain becomes strong with less poleward turning of the velocity vectors in the presence of an equatorward wind that reduces (or stops) the downward velocity component due to diffusion and raises the ionosphere to high altitudes of reduced chemical loss. The EIA crests in peak electron density and total electron content shift rapidly to higher than normal latitudes during the PPEF event. However, the crests become stronger than normal only in the presence of an equatorward wind. The results suggest that the presence of an equatorward neutral wind is required to produce a strong positive ionospheric storm during a daytime eastward PPEF event. The equatorward neutral wind need not be a storm time wind though stronger wind can lead to stronger ionospheric storms.

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