A new type of the additional layer is predicted to occur in the low-latitude F region ionosphere during evening hours of a major magnetic storm. By using the coupled NCAR Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) and Sheffield University Plasmasphere Ionosphere Model (SUPIM) runs during the major magnetic storm of 29–30 October 2003, we present a new type of additional layer which occurs at the equatorial ionization anomaly (EIA) by a new physical mechanism. This mechanism requires that storm time meridional neutral wind surges travel from high to low latitudes and cross into the opposite hemisphere. The wind surges modify the field-aligned plasma velocities in the EIA regions significantly after uplift of the ionospheric layer by a penetration electric field and interact with the downward field-aligned plasma velocities of the enhanced equatorial fountain. The combined storm effects of the enhanced plasma fountain and the neutral wind surges result in plasma convergence in altitude and form the additional layers underneath the EIA crests.
 In the low-latitude and equatorial ionosphere, an additional layer of the F region, named the F3 layer, has been studied theoretically [Balan and Bailey, 1995; Balan et al., 1997, 1998; Jenkins et al., 1997] with supporting evidence from ground-based radar observations [Balan et al., 1997, 1998; Jenkins et al., 1997; Hsiao et al., 2001; Lynn et al., 2000; Rama Rao et al., 2005; Uemoto et al., 2007] during magnetically quiescent periods. According to Balan et al. , the F3 layer occurs mainly during the morning-noon (∼0830–1630 LT) period due to the combined effect of the upward E × B drift and the neutral wind that provides vertically upward plasma drifts at and above the F2 layer. The upward drifts raise the F2 layer to become the F3 layer while a new F2 layer forms at the original F2 altitude. During daytime, the peak electron density of the F3 layer exceeds that of the new F2 layer and, therefore, an ionosonde is able to observe both layers. Because of the neutral wind effect in supporting the additional vertical uplift at off-equatorial locations, the F3 layer is predicted to be most prominent on the summer side of the magnetic equator [Balan et al., 1998; Rama Rao et al., 2005]. In addition to being seen either at the magnetic equator or in the low-latitude summer hemisphere, the F3 layer can also be seen in both regions simultaneously. Uemoto et al.  show ionosonde observations of the F3 layer in both the winter and summer hemispheres concurrently. This is explained by plasma diffusion from the magnetic equator to the low-latitude regions due to the pressure gradient and the gravitational force.
 The F3 layer not only occurs during quiescent ionosphere periods but also during magnetic storm periods. Recent observations show the existence of a storm time F3 layer occurring at the magnetic equator in response to a strong penetration electric field [e.g., Zhao et al., 2005; Paznukhov et al., 2007; Balan et al., 2008]. The mechanism responsible for the storm time F3 layer is similar to that in quiet periods but with a much faster processing time due to the rapid uplift of the F layer by an upward E × B drift resulting from an eastward penetration electric field. A theoretical model of this effect is presented in detail by Lin et al. . To see whether the F3 layer also occurs at off-equator low-latitude locations and to study the associated physical mechanism, we extend the theoretical model runs of Lin et al.  for the same intense storm period of the 29–30 October 2003. A new type of additional layer occurring in the EIA crest region during nighttime is predicted by the extended model runs during the storm period, and the associated physical mechanism is examined in this paper.
2. Model Description
 In this study, The National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere General-Circulation Model (TIEGCM) [Roble et al., 1988; Richmond et al., 1992] is used for simulating the global thermosphere-ionosphere responses to the 29–30 October storm event. The Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure [Richmond and Kamide, 1988], which has inputs from both ground-based and satellite measurements, is used to specify the auroral precipitation and high-latitude convection [cf. Lu et al., 1998]. Since the TIEGCM has an upper boundary at around 800 km altitude, an ionospheric model that can simulate the low-latitude and midlatitude ionosphere to greater altitudes is desirable for simulating the enhanced EIA during the storm. We therefore run the Sheffield University Plasmasphere Ionosphere Model (SUPIM), using the TIEGCM neutral winds, temperature, and composition as inputs.
 SUPIM is used to simulate the low-latitude ionosphere and plasmasphere [Bailey and Sellek, 1990; Bailey and Balan, 1996]. The model solves the coupled time-dependent equations of continuity, momentum, and energy for the electrons and the O+, H+, He+, N2+, NO+, and O2+ ions, using the implicit finite difference method along closed eccentric dipole geomagnetic field lines. In this simulation, 108 field lines with apex altitudes distributed from 150 km to 25,000 km are used. Since SUPIM is a two-dimensional model that only simulates the plasmasphere and ionosphere in a single longitudinal sector, we simulate the ionospheric responses to the storm event at −70° geographic longitude (∼4° magnetic longitude) where the great TEC enhancements are observed by Lin et al. [2005a]. In the calculation of the plasma E × B drifts, SUPIM uses a semi-Lagrangian method that allows field lines to move vertically and meridionally with the E × B drifts and interpolates back to a fixed coordinate at each time step. The TIEGCM neutral composition, temperature, and winds are interpolated to the SUPIM grid at each time step. Above the TIEGCM upper boundary at pressure level +7 (∼800 km in altitude in this simulation), the neutral winds and temperature are assumed to equal their upper boundary values, while the densities of N2, O2, and O are assumed to be in diffusive equilibrium. The E × B drifts during 29–30 October 2003, derived from ROCSAT-1 measurements, as described by Lin et al. [2005b] and presented here in Figure 1, are used to specify field line convection in SUPIM between 200 and 25,000 km altitude with the same approach as that described by Lin et al. [2005b].
 The simulation performed in this study is similar to that of Lin et al. , considering the storm time E × B drifts derived from ROCSAT-1 observations and the storm time neutral winds from the NCAR-TIEGCM runs. Since the neutral composition changes mainly act to decrease the electron density above ±35° magnetic latitude and increase it in the low-latitude and equatorial regions without changing the general plasma structure, the quiet time neutral composition output from TIEGCM run is incorporated into the model instead.
3. Model Results and Discussions
Figure 1 shows the scaled E × B drifts at 300 km during 29–30 October 2003. On 29 October, the storm-produced upward E × B drift is most significantly seen between 1800 and 2200 UT. The disturbed upward E × B drift ceased after 2200 UT on 29 October and showed magnitude similar to the quiet time E × B drift until 0300 UT on 30 October. Later, a disturbed upward drift is enhanced slightly between 0300 and 0600 UT. The additional layer predicted by this study mainly occurs after the strong storm-produced uplift where the E × B drifts return to similar quiet time magnitudes.
Figure 2 shows the prestorm neutral wind patterns at the F region altitudes (∼300 km) modeled by the NCAR TIEGCM, while Figure 3 shows the storm time neutral wind patterns from 2200 to 2400 UT on 29 October (DOY 302). Comparing Figures 2 and 3, it indicates that equatorward neutral wind surges are launched from the northern and the southern high-latitude regions due to storm time energy and momentum deposition. The equatorward neutral wind surges are most clearly seen in the nighttime region. Figure 3 shows that the equatorward wind surges travel from high to low latitudes and cross over each other around the geographic equator. After the crossover, the wind surge from each hemisphere then travels from low to high latitudes in the opposite hemisphere, with a poleward wind. During the storm period, similar neutral wind surges are launched from high latitudes and propagate globally, occurring continuously and resulting in a complex neutral wind pattern, especially during evening hours [e.g., Lu et al., 2001]. To assess the impact of the winds on the ionospheric plasma structure in addition to the electric field effects, two simulations are performed with SUPIM. Simulation case 1 incorporates both the storm time E × B drift and the neutral winds, while simulation case 2 incorporates the storm time E × B drift but the quiet time neutral winds instead. Figure 4 shows results from case 1 in Figure 4 (left) and case 2 in Figure 4 (right). A clear double layer signature is seen underneath each of the EIA crests in the case 1 results, while only a single-layer EIA crest feature located at a lower altitude is seen in case 2. The double layer signature could be recognized as an ionospheric additional layer by a ground-based ionosonde when the higher layer shows a greater electron density than the lower layer.
 To further examine the neutral wind effects on the additional layer formation in detail, we plot the neutral meridional winds, O+ field-aligned velocities, and plasma density between 100 and 1000 km altitude in Figures 5–8 from 2230 UT (1750 LT) on 29 October to 0630 UT (0150 LT) on 30 October. The electron densities are shown half an hour later since it normally takes some time for them to respond to the neutral winds and plasma velocities. Meanwhile, blue and red arrows are drawn to indicate the O+ flow direction schematically. It is seen in Figure 5 that the equatorward wind surges from both hemispheres cross each other at 2300 UT (1820 LT) and become poleward wind surges at 2330 UT (1850 LT). It is important to note that although the wind surges above 400 km altitude are crossing over and changing from equatorward to poleward, the wind patterns below 400 km remain in the equatorward direction. The O+ field-aligned velocities are then changed significantly due to the complex neutral winds, especially below 600 km altitude, where the ion-neutral collision frequency is much greater than at higher altitudes. At 2230 UT (1750 LT) in Figure 5, the O+ field-aligned velocities are downward in both northern and southern hemispheres above 600 km, while the O+ field-aligned velocities are upward below 600 km. The downward velocities above 600 km result from the equatorial plasma fountain effect associated with the plasma uplift at the magnetic equator. On the other hand, the off-equator upward velocities below 600 km are produced by the equatorward neutral winds that lift the plasma upward along the magnetic field lines. The EIA crests are then situated in regions near 600 km altitude where the downward and upward O+ flows converge at 2300 UT (1820 LT). At 2330 UT (1850 LT), the neutral winds above 400 km become poleward, while the winds below remain equatorward. The O+ field-aligned velocities, produced by fountain effect, remain downward (poleward) above 700 km. Below 700 km, the velocities show upward (equatorward), downward (poleward), and upward (equatorward) directions at around 600, 400, and 200 km altitudes, respectively. The ionospheric plasma is then expected to converge at 700 km and 300 km due to the field-aligned velocities. The electron density plot at 2400 UT (1920 LT) shows the feature of plasma dripping downward to lower altitudes around 400 km. This feature becomes clearer in Figure 6. At 0030 UT (1950 LT), stronger downward velocities are seen between 600 and 200 km altitudes, which results in the plasma at the EIA crests dripping downward at 0100 and 0130 UT (2020 and 2050 LT). The double layer structure becomes more and more prominent after 0130 UT. The neutral winds turn from poleward to equatorward again at 0130 UT and the lower layers are then sustained at 400 km altitude by the equatorward winds. An ionosonde can observe the double layer structure most significantly at 0130 and 0200 UT (2050 and 2120 LT), since the electron density in the lower layers is smaller than in the upper layers. After 0230 UT (Figure 7), the lower layers are now well developed, and the electron densities become larger than in the upper layers as the electron density continues to be transported downward along magnetic field lines. This process lasts until 0630 UT on 30 October (DOY 303), and another similar double layer structure is formed at lower altitudes at a later time along with the descending EIA crests. Figure 8 shows the neutral winds and O+ field-aligned velocities from 0530 UT (0050 LT) to 0630 UT (0150 LT) and the corresponding ionospheric electron density from 0600 to 0700 UT. At 0700 UT, a double layer feature is again formed below the EIA crests by the downward field-aligned plasma velocities. The double layer structure lasts until 0800 UT on 30 October, when the E × B drift turns from downward to upward.
Figure 9 shows line plots of the field-aligned O+ velocities during 0030–0200 UT and the corresponding electron densities 30 min later at 23°N geographic latitude (∼32°N magnetic latitude) where the additional layer signature is clearly seen in the northern hemisphere. From Figure 9, the field-aligned O+ velocities are in northward direction (or poleward direction at this location) with two stronger flow velocities located at 1000 and 400 km altitudes at 0030UT (1950 LT). The northward velocities are weakened from 1000 km to 600 km altitude where the corresponding F2 peak (0100 UT) is located at. In later time, at 0100 UT (2020 LT), the northward O+ velocities between 200 and 400 km altitudes become smaller and the additional layer formation is more discernible at 400 km altitude at 0130 UT. The O+ velocities between 200 and 400 km altitudes turn from northward (poleward) to southward (equatorward) at 0130 UT and result in intensification of the additional layer below the F2 peak. The electron density plot at later time, 0200 UT, indicates that the magnitude of the additional layer electron density is similar to that of the F2 peak electron density. With continuing southward (equatorward) turning of the O+ velocities between 200 and 400 km altitudes at 0200 UT, the additional layer electron density becomes greater than the F2 peak electron density at 0230 UT. The southward (equatorward) turning of the original northward (poleward) O+ velocities between 200 and 400 km altitudes shown in Figure 9 indicates again that the formation of the additional layer underneath the F2 peak results from altitudinal variations of the O+ velocities.
 Unlike the complex neutral wind patterns and the field-aligned velocities shown in the model run with inputs of both the storm time electric field and neutral winds, the O+ velocities and the electron density structures are much simpler in the run having inputs of storm time E × B drift but quiet time neutral winds (Figure 10). In Figure 10, the downward plasma velocities are formed by the enhanced plasma fountain effect produced by the storm time eastward penetration electric field. Poleward extended and enhanced EIA crests are seen in Figure 10, indicating the super fountain effect [cf. Tsurutani et al., 2004; Lin et al., 2005a, 2005b; Anderson et al., 2006]. The EIA crests are located at lower altitude than those in Figures 5–8, since there is no equatorward meridional wind to maintain elevated EIA crests as in the mechanism described by Lin et al. [2005b].
4. Summary and Conclusion
 The model simulations preformed in this study by one-way coupling of the NCAR TIEGCM and the SUPIM show the importance of the storm time meridional neutral wind in producing an additional ionospheric layer underneath the EIA crest during the evening hours of a major magnetic storm. The new type of additional layer predicted by this paper is formed by combing the effects of a strong equatorial plasma fountain and equatorward meridional neutral wind surges launched from high latitudes and traveling to the equator and beyond. The neutral wind surges modify the morphologies of the field-aligned plasma flows of the plasma fountain and create plasma convergence in altitude, resulting in additional layer formation. This additional layer appears at latitudes away from the magnetic equator in the absence of significant photoproduction and thus has a different formation that previously described by Balan et al. . Previously, the original F peak is moved upward and an additional peak is produced below it. In the case discussed here, the additional layer is produced below the original peak by the action of equatorward neutral winds that serve to prevent the downward diffusion of ions transported away from the equator by upward E × B drifts.
 Part of this work was carried out when C.H.L. was a Newkirk Graduate Research Associate at NCAR/HAO, with support from the NASA Sun-Earth Connection Theory Program. G. Lu was supported in part by NASA's Heliophysics Guest Investigator program. This work is partly supported by Taiwan National Science Council under grants NSC 97-2111-M-006-003 and NSC 98-2111-M-006-003-MY2 and by National Space Organization (NSPO) under 98-NSPO(B)-IC-FA07-01(V). The National Center for Atmospheric Research is sponsored by the National Science Foundation.
 Zuyin Pu thanks Jiuhou Lei and another reviewer for their assistance in evaluating this paper.