Recently, nighttime ultraviolet (UV) observations obtained by IMAGE FUV and TIMED GUVI instruments have revealed a longitudinal wave number four pattern in the nighttime airglow intensity and in the position of the equatorial anomalies during equinox and high solar flux conditions. In the present study, we have extended this work and determined the longitudinal variability of the low-latitude total electron content (TEC) climatology during different geophysical conditions with a special emphasis on the longitudinal wave number four structure in the low-latitude ionosphere. We have used more than 5 million low-latitude TOPEX TEC observations covering the entire 13 years of TOPEX TEC data from August 1992 until October 2005. This data set was used to determine the local time, seasonal, solar cycle, and geomagnetic activity dependence of the longitudinal variability of TEC at equatorial and low latitudes, and in particular, to address the existence and evolution of the wave number four longitudinal pattern under these conditions. Our study shows that the wave number four pattern is created during the daytime hours at equinox and June solstice but is absent, or washed out by other processes, during December solstice. During equinox the wave number four pattern is created around noon with well-defined longitudinal enhancements in the low-latitude TEC. These enhancements, which are symmetric about the geomagnetic equator during this season, last for many hours and can be clearly seen past midnight. The longitudinal patterns are found to be nearly identical between the vernal (March/April) and autumnal (September/October) equinoxes and largely independent of the solar cycle conditions. The wave number four pattern is also observed during geomagnetically active conditions, indicating that the processes that create this pattern are also present during active times. The variations between the well-defined longitudinal maxima and minima are of the order of 20%. During June solstice, the wave number four pattern is also observed in the afternoon hours but, in contrast to the equinox cases, it exhibits a strong hemispheric asymmetry and is not observed during the night. The low-latitude TEC exhibits clear longitudinal variations during December solstice, with large daytime enhancements over the east Asian and Pacific regions and a third enhancement emerging in the afternoon over the Atlantic Ocean, but a clear wave number four pattern is not observed during this season. Although the equatorial and low-latitude TEC values exhibit clear longitudinal patterns during all seasons, a significant amount of scatter remains in the TEC data that is not accounted for by changes in the solar cycle, the season, or the local time or by the longitudinal variability. This remaining scatter is largest near the poleward edges of the anomalies and is of the order of 40%.
 The low-latitude ionosphere is a highly dynamic environment that exhibits significant variations with local time, altitude, latitude, longitude, solar cycle, season, and geomagnetic activity. This large variability arises from the couplings, time delays, and feedback mechanisms that are inherent in the ionosphere-thermosphere system, as well as from effects of solar, interplanetary, magnetospheric, and mesospheric processes. Ionospheric electric fields play important roles on the plasma distribution and dynamics of the equatorial and low-latitude ionosphere. At equatorial latitudes, ionospheric electric fields drive the F region plasma motion and, in combination with the gravitational force and pressure gradients, largely affect the development of the Appleton anomalies [Appleton, 1946], a term used to describe the ionization crests that are located at about 10 to 15 degrees on each side of the geomagnetic equator. Thermospheric neutral winds can further influence the development of these anomalies and can lead to asymmetries in the northern and southern peak density values.
 TEC measurements from the TOPEX/Poseidon mission [Fu et al., 1994] during the last decade have provided an excellent database over the ocean areas, where conventional measurements are sparse, to determine the global ionospheric climatology and to investigate its variability. The global TEC climatology has been studied by Codrescu et al. [1999, 2001] using TOPEX TEC data obtained during low solar flux conditions. Jee et al.  have extended these studies by using almost 10 years of TOPEX TEC data, which also included data from periods of high solar flux conditions. In their study, the global TEC climatology for three seasonal conditions (equinox, December solstice, and June solstice) and three longitude domains (Pacific, Indian, and Atlantic Oceans) was established. Their study of the longitudinal TEC variability, however, was focused on midlatitude regions and no clear longitudinal variability pattern at low-latitudes was determined.
Vladimer et al.  have used TOPEX TEC data from 1993 to 1995 to determine longitudinal variations in the afternoon equatorial anomaly during low solar flux conditions. They noted a relative enhancement in the afternoon anomaly TEC values in the Indian/Asian longitude sector. These enhancements were seen primarily in the Asian southern hemisphere. Furthermore, they observed a decrease in the anomaly TEC values in the western American region. Traditionally, these longitudinal variations in the low-latitude plasma distributions have been attributed to (1) the offset of the geomagnetic equator from the geographic equator, (2) variations in the magnetic declination angle, and (3) variations in the electric and magnetic fields [Walker, 1981].
 Recently, Immel et al.  have used nighttime UV observations from the IMAGE and TIMED satellites to study the longitudinal variation of the low-latitude ionosphere during 30 d in March/April of 2002. They have revealed a global wave number four longitudinal signature of the nighttime airglow intensity and in the position of the equatorial anomaly. They explained the observed nighttime variations as the result of longitudinal variations in the diurnal amplitude of atmospheric tides in the daytime E region ionosphere and a corresponding modulation of the daytime zonal electric field. Their nighttime observations, however, could not directly confirm that the observed longitudinal variations are not the result of a modulation in the F region wind near the dusk terminator and/or the result of a longitudinal modulation in the prereversal enhancement of the F region vertical drift. The prereversal drift is generally considered to be an important factor for the morphology of the nighttime plasma density at low latitudes. England et al. [2006b] have used noontime equatorial electrojet observations from the CHAMP, Orsted, and SAC-C satellites and demonstrated that a similar wave number four variation exists in the daytime electrojet sheet current density during geomagnetically quiet periods near equinox. Hartman and Heelis  have used 2 years (2001–2002) of ion drift measurements from the ion drift meter on board the Defense Meteorological Satellite Program (DMSP) F15 satellite to examine the longitudinal variations in the vertical ion drift at the dip equator in the 0930 local time sector. Their study indicated that the wave number four longitudinal variations appear to be present throughout the year but are more influential during equinox conditions.
 In this study we used the entire 13 years of TOPEX TEC data (August 1992 to October 2005) to determine the local time, seasonal, solar cycle, and geomagnetic activity dependence of the longitudinal variability of the TEC at equatorial and low latitudes. Of particular interest in this study was the existence and evolution of the wave number four longitudinal signature in the TEC data under different geophysical conditions. In what follows we first describe our TEC database as well as the data preparation procedures. Next, an analysis of the overall longitudinal variability of the low-latitude TEC values during equinox, June solstice, and December solstice conditions will be presented. Finally, the dependence of the longitudinal variability on the solar cycle and geomagnetic activity conditions will be established, with an emphasis, again, on the dependence of the wave number four structure of the low-latitude ionosphere on the different geophysical conditions.
2. Measurements and Data Preparation
 The TOPEX/Poseidon mission is a joint mission of the National Aeronautics and Space Administration (NASA) and the French space agency, Centre National d'Etudes Spatiales (CNES) to study the surface height of the Earth's oceans. The satellite was launched on 10 August 1992 [Fu et al., 1994] and remained operational until October 2005. TOPEX/Poseidon orbits the Earth at an altitude of 1336 km with an inclination angle of 66° and a period of 112 min. Consecutive orbits are offset to the west by about 30° longitude. There are 127 orbits in each 9.916 d period, which means that the satellite passes vertically over the same location, to within 1 km, about every 10 d. The satellite orbits are close to Sun-synchronous, advancing by 2° per day. Consequently, it takes about 90 d to cover all local times. This slow precession of the orbit, however, enables the satellite on any given day to observe TEC at approximately the same local time across the globe in 30° longitude steps.
 TOPEX/Poseidon uses a dual-frequency radar altimeter operating simultaneously at 13.6 GHz (Ku band) and 5.3 GHz (C band) to observe the surface height of the oceans. Because of the dispersive properties of the ionosphere, the observations at the two frequencies also provide a direct estimate of the electron content along the ray path from the satellite to the surface of the ocean. The measured electron content is essentially equivalent to the total electron content (TEC) of the ionosphere in a vertical column extending from the subsatellite reflection point on the surface of the ocean to the height of the satellite at 1336 km altitude.
 The TEC data used in this study were obtained from the NASA Physical Oceanography Distributed Active Archive center at the Jet Propulsion Laboratory (JPL PO.DAAC/NASA). The TEC measurements were taken at a rate of about one observation per second. For this study we averaged the 1-s data into 18-s averages corresponding to about 1° in satellite orbit (i.e., each TEC value was obtained by averaging the data for 18 s). The scatter of the 1-s TEC values about the 18-s averaged mean shows a fairly constant spread of about 4 to 5 TECU (1 TECU = 1016 electrons/m2) for different seasons, local times, and hemispheres [Jee et al., 2004]. For each 18-s data point the corresponding geomagnetic latitude, longitude, and magnetic local time (MLT) was calculated using quasi-dipole coordinates [Richmond, 1995]. Since the focus of this study is on the TEC variability at equatorial and low-latitudes, only data that were observed in a ±30° geomagnetic latitude range about the geomagnetic equator were used in our analysis.
 Our TEC database covers the entire 13 years of the TOPEX/Poseidon mission from August 1992 until October 2005. This period spans more than one entire solar cycle (Figure 1). The total number of 18-s averaged data points in the ±30° geomagnetic latitude range for the 13 years that were used in this study is 5,110,842.
Figure 2 shows an example of a 3-d period of TEC observations using only data from the ascending node of the satellite (when the satellite crossed from the southern to the northern hemisphere). The TEC values were obtained from day 89 to 91 of 2002, which is when the satellite crossed the geomagnetic equator at about 2100 magnetic local time. This time period also corresponds to the date and local time period of the IMAGE FUV observations previously discussed [Immel et al., 2006]. Clearly seen is the Appleton anomaly with large bands of enhancements in TEC (magenta color) centered about the geomagnetic equator at a distance of about 10° to 15° in latitude. The density peaks shown in Figure 2 vary from about 60 to more than 120 TECU and exhibit fluctuations in their latitudinal separation from the geomagnetic equator (shown as the thick black line in Figure 2). In particular, the density peaks are larger and further away from the geomagnetic equator in the Pacific region near 200°E longitude and are smaller and closer to the equator at a location that is only about 40° to the east (at about 240°E longitude). Over the Atlantic, a large hemispheric asymmetry in the density peaks can be seen with the larger peaks in the southern hemisphere. This asymmetry is most likely due to the effects of neutral winds in this region.
 Owing to the orbit of the TOPEX/Poseidon satellite, difficulties arise for a statistical analysis of the longitudinal variability of the TEC values. Because of the slow precession of the satellite orbit (2°/d), the local time coverage of the observations changes by about 4 h over the course of 1 month. As a result, TEC observations near the beginning of the month can differ significantly from those obtained toward the end of the month simply due to the differences in local time coverage. Therefore in a study of the longitudinal TEC variability, the TOPEX TEC data cannot simply be averaged over month-long periods. Furthermore, TEC observations made during different phases of the solar cycle cannot easily be combined in a statistical analysis of the longitudinal variability due to the large dependence of the TEC values on the solar flux conditions [e.g., Jee et al., 2004]. To circumvent this problem, we first normalized the TEC values to a common baseline. This then allowed us to combine TEC observations obtained during different geophysical conditions.
 We used daily values of the longitudinally averaged low-latitude peak TEC values as this baseline. Specifically, for each individual ascending pass (when the satellite crossed form the southern to the northern hemisphere) and each individual descending pass (when the satellite crossed from the northern to the southern hemisphere) of the satellite we determined the peak TEC value in the ±30° geomagnetic latitude region. During most times, these peak values correspond to the maximum TEC values found in the Appleton peaks. However, during early morning hours, when the anomaly peaks are weak, the locations of these maximum TEC values are not necessarily related to the anomaly peaks but instead can be related to localized regions of nighttime maintenance (possibly due to neutral winds) and/or the location of the dawn terminator. Next, the individual peak values were used separately for the ascending and descending phase of the satellite orbit to create longitudinally averaged daily peak values. These daily values were calculated by averaging the individual peak values over 3-d-long periods centered on the respective day. Figure 3 shows the daily values for the entire 13-year database for both the ascending and descending phase of the satellite. The strong fluctuations in the peak values are due to their seasonal, solar cycle, and local time dependence. Note that due to the inclination of the TOPEX orbit the satellite traverses the northern and southern anomalies at slightly different local times (see Figure 2). Depending on the phase of the orbit (ascending or descending) the northern hemisphere is traversed later or earlier in local time and this could lead to a hemispheric bias in our normalization factors. However, since the results of our statistical analysis will correspond to many years of TOPEX observations and represent averages over both ascending and descending phases of the satellite orbit the local time differences will be averaged out.
 Next, the daily values shown in Figure 3 were used as the normalization factors in our study and each individual 18-s TEC data point was divided by its corresponding normalization factor. The net effect of this normalization is a minimization of the effects of local time and solar flux on the TEC values. As a result, normalized TEC values from different phases of the solar cycle and different local times can be combined in order to statistically investigate the longitudinal variability of the low-latitude TEC. This enables us to directly compare the variations during different geophysical conditions.
 The advantages of the normalization become even more clear in Figure 4. The top panels show a scatterplot of all TEC values obtained in a 10°-wide longitudinal region from 230° to 240°E longitude during equinox conditions (March/April, September/October). The data correspond to a 2-h wide magnetic local time range from 1400 to 1600 MLT and are shown separately in the left panel for low solar flux conditions (F10.7cm < 130) and in the right panel for high solar flux conditions (F10.7cm > 130). The average F10.7cm flux values for the two bins were 91 and 178, respectively. Superimposed on the scatter plots are the median values obtained in 1°-wide latitude bins and the first and ninth decile shown as error bars. It can clearly be seen that the TEC values differ significantly between the two solar flux conditions and increase by about a factor of 2 (100%) from low to high solar flux conditions. The scatter about the median values is similar for the low and high solar flux conditions and is of the order of 10 to 40 TECU with the larger values occurring near the anomaly peaks. In the bottom panels the corresponding normalized TEC values are shown. There are several points that can be noted. First, the normalized values are very similar for the two solar flux cases and the median values only differ slightly. Second, the scatter in the data has been significantly reduced, in particular near the geomagnetic equator. Third, the median values are all less than one, indicating that values in this longitude range are smaller than the zonally averaged peak TEC values. However, it can also be seen that there is still a significant amount of variability in the normalized TEC values, especially on the poleward slopes of the anomaly peaks. This scatter is most likely due to variations in both the equatorial vertical drifts from one day to the next, which places the peaks at slightly different latitudes, and the effects of neutral winds, which change the distribution of the ionization along the magnetic field lines. As a net effect, this has the tendency to somewhat smear out the peak median values in our analysis.
 To examine the longitudinal TEC morphology, we created two-dimensional maps of the median relative TEC variations. These maps were created by first sorting the normalized data according to their geophysical conditions (e.g., season, solar flux, magnetic local time, geomagnetic activity) and then binning them every 10° in longitude into 15° × 1° bins in geomagnetic longitude and geomagnetic latitude. Here we have used overlapping longitudinal bins in order to both increase the number of data points in each bin (in particular near landmasses) and to provide for an improved representation of longitudinal gradients. Finally, in each bin the median values were calculated. In what follows these maps will be discussed.
3. Results and Discussion
3.1. Seasonal Variations
 The low-latitude ionosphere TEC morphology exhibits large seasonal variations [e.g., Codrescu et al., 1999, 2001; Jee et al., 2004] due to seasonal changes in the equatorial vertical drift, in the strength and direction of the meridional neutral wind, in the neutral composition, and in the solar illumination of the northern and southern hemispheres. At equinox, the TEC pattern is largely symmetric about the geomagnetic equator, but large asymmetries are observed during solstice conditions [e.g., Jee et al., 2004]. This basic morphology can be understood by examining the solar illumination in the southern and northern hemisphere and the direction of the neutral meridional wind. During the solstices, the neutral wind blows from the summer to the winter hemisphere, raising the F region ionization in the summer hemisphere and lowering it in the winter hemisphere [Schunk and Nagy, 2000]. This wind effect, coupled with the asymmetry in the solar ionization and neutral composition in the two hemispheres, accounts for the observed asymmetries during the solstices [e.g., Liu et al., 2007]. Furthermore, seasonal variations in the equatorial vertical drift [Fejer et al., 1995; Scherliess and Fejer, 1999] lead to differences in the strength and location of the Appleton anomaly peaks from season to season.
 Consequently, in our analysis, we separated the normalized TEC data into their respective seasons. In detail, the data were separated into three 4-month long seasonal bins; Equinox (March–April, September–October), June solstice (May–August), and December solstice (November–February). In our initial analysis of the longitudinal TEC variability, data from all solar flux conditions were used. In order to minimize effects associated with geomagnetic activity, only data obtained during extended geomagnetically quiet conditions were used. For this analysis, geomagnetically quiet periods were defined when the average Kp index was below 3.0 for the 9 h prior to the time of the observation. The resulting data were binned into eight 3-h long magnetic local time bins (0000–0300 MLT, 0300–0600 MLT, up to 2100–2400 MLT) and into 15° × 1° geomagnetic longitude/latitude bins every 10° in longitude. Finally, in each of the 51,840 bins (3 seasons × 8 MLTs × 36 longitudes × 60 latitudes) the median value was calculated. In addition, the corresponding average values were also calculated for each bin, but no significant differences between the results using medians or averages were found. Also note that our criteria for geomagnetically quiet periods might not exclude perturbations in the low-latitude electric fields and neutral composition and winds associated with periods of extended geomagnetic activity [Fuller-Rowell et al., 1994; Scherliess and Fejer, 1997]. An extension of our Kp criteria from 9 to 36 h, however, did not result in significant differences in our median values.
Figures 5a–5c show the median normalized TEC values as a function of geomagnetic latitude and longitude for equinox (Figure 5a), June solstice (Figure 5b), and December solstice (Figure 5c). Each figure is separated into eight panels corresponding to the eight magnetic local time bins starting in the upper-left panel near sunrise (0600–0900 MLT) and ending in the lower-right panel in the late night (0300–0600 MLT). In order to better geolocate the observed features on these maps, we shifted the geomagnetic longitude of each bin by 69° to the east. This applied shift approximately colocates the 0° longitude with the geographic 0°-meridian. The white areas in Figures 5a–5c indicate that TOPEX did not obtain observations in these longitude/latitude bins associated with its traversing over the continents. The color scale used for the normalized TEC maps ranges from 0.4 to 1.2 for all panels.
 The most prominent features seen in Figures 5a–5c are strong enhancements in the relative TEC variations that exhibit significant longitudinal variations. During equinoxes (Figure 5a) two separate enhancements in relative TEC develop after sunrise (0600–0900 MLT) over east Asia between about 80°–120°E longitude and over the Pacific Ocean at 170°–270°E longitude. Initially, these enhancements form near the geomagnetic equator and extend up to about ±10° in geomagnetic latitude on both sides of the equator. Over the Atlantic ocean, between about 300°E to 0°E longitude, these enhancements are absent after sunrise, indicating that the TEC values over the Atlantic are significantly smaller (≈20%) than the TEC values in the Pacific region at this local time.
 In the next panel, which shows the normalized TEC values 3 h later (0900–1200 MLT), the two equatorial enhancements over the east Asian and the Pacific region have moved poleward in both hemispheres and have separated into northern and southern enhancements corresponding to the development of the equatorial anomaly at these local times. Furthermore, the strong enhancement over the Pacific Ocean has began to split into two separate longitudinal enhancements on both sides of the equator located at about 170°–200°E and 230°–270°E longitude, separated by a local minimum located near 220°E longitude (more clearly seen in the southern hemisphere).
 This longitudinal separation becomes even more evident in the next panel from 1200 to 1500 MLT. At this time, the equatorial anomaly peaks are clearly established and nearly symmetrically located on both sides of the geomagnetic equator at about ±15° geomagnetic latitude. Three separate enhancements on both sides of the equator can clearly be identified over the east Asian and the Pacific sectors with their maxima located near 100°E, 190°E, and 270°E longitude. A fourth enhancement is also now evident over the Atlantic. The strongest enhancement at this time is over the east Asian region and the weakest enhancement is over the Atlantic.
 At 1500–1800 MLT the four enhancements continue with a strengthening over the Atlantic Ocean. The variations between the four maxima and their neighboring minima is of the order of 20–30%, which is similar to the relative variations between the anomaly enhancements and the equatorial valley regions. After sunset (1800–2100 MLT), the four peaks in both hemispheres are still clearly observable but are starting to diminish in peak strength. This weakening of the peak values is largely due to a smearing of the peaks due to a dependence of the geomagnetic latitude extent of the anomaly on the solar flux conditions at these local times. This can be understood by examining the dependence of the equatorial vertical drift on the solar flux conditions. During daytime, the equatorial vertical drift, which is the main driver of the anomaly, is nearly independent of the solar flux conditions [e.g., Fejer et al., 1995; Scherliess and Fejer, 1999], and as a result the locations of the anomaly peaks appear at nearly the same geomagnetic latitudes during low and high solar flux conditions. During the evening hours, however, the prereversal enhancement in the equatorial vertical drift as well as the nighttime drifts exhibit a strong solar flux dependence, resulting in a significant solar flux variation in the locations of the anomaly peak values. This leads, when data from different solar flux conditions are combined (as done in Figure 5a) to a washed out map of the enhancements. In section 3.2, we separate the equinoctial data into low and high solar flux conditions, which provides a better representation of the four low-latitude enhancements during the night.
Immel et al.  used nighttime UV observations from the IMAGE satellite to study the longitudinal variation of the low-latitude ionosphere during 30 d in March/April of 2002. Their study revealed a global wave number four longitudinal pattern of the nighttime airglow intensity associated with the equatorial anomaly in agreement with our results. They speculated that the observed nighttime variations are the result of longitudinal variations in the amplitude of the diurnal atmospheric tides in the daytime E region ionosphere and a corresponding modulation of the daytime zonal electric field. Their nighttime observations, however, could not directly confirm that the observed longitudinal variation was not the result of a modulation in the F region wind near the dusk terminator and/or due to a longitudinal modulation in the prereversal enhancement of the F region vertical drift near dusk. England et al. [2006b] demonstrated that the wave number four pattern also exists in the daytime electrojet sheet current density, which provided further evidence for the importance of the daytime ionospheric E region in generating these patterns. Their study, however, could not directly relate their daytime E region observations to the nighttime F region UV observations. Our results clearly show that the observed wave number four longitudinal structure in the low-latitude plasma density is established during the daytime hours and is not the result of a longitudinal modulation in the prereversal enhancement near dusk. This strongly supports the interpretation that the wave number four pattern is the result of a longitudinal variation in the E region daytime tidal amplitudes and a resulting longitudinal variation in the daytime equatorial zonal electric field (vertical drift).
 The Immel et al.  study also reported a phase shift in the peak brightness of their nighttime UV observations of about 20° to the east when compared to the diurnal peak amplitudes of the temperature variation at 115 km as given by the Global Scale Wave Model (GSWM) [Hagan et al., 2001; Hagan and Forbes, 2002, 2003; Forbes et al., 2003]. A detailed inspection of Figure 5a reveals that the locations of our nighttime enhancements are shifted by about 20° to the east from the 1500–1800 MLT bin to the 2100–2400 MLT bin. For example, the peaks located near 100°E longitude and 270°E longitude at 1500–1800 MLT are shifted toward 120°E longitude and 290°E longitude at 2100–2400 MLT, respectively. From our analysis it is not possible to determine if this shift is the result of a virtual drift of the peaks (e.g., a decay on the western crest and a buildup on the eastern side) or due to an actual drift of the entire enhancement to the east. We speculate that this drift is the result of an upward-poleward electric field, causing the plasma to drift to the east, which is similar to the movement of low-latitude plasma bubbles during the night [Immel et al., 2003; Makela, 2006]. The eastward plasma velocity required to move the plasma over a 20°-longitude region in a 6-h period is approximately 100m/s, which corresponds to typical zonal drift velocities at equatorial and low-latitudes in the evening hours [Fejer et al., 1991].
Figure 5b shows the normalized TEC values for June solstice conditions. The data correspond again to the mean values during moderate geomagnetic activity and all solar flux conditions. Similar to the development during the equinoxes, two separate enhancements develop after sunrise (at 0600–0900 MLT), one over east Asia (70°–120°E) and one over the Pacific Ocean (150°–260°E). During the next several hours, the Pacific enhancement separates in both hemispheres into two distinct peaks, which are located near 190° E and 260°E longitude at 1200–1500 MLT. However, in contrast to the equinoctial case, these two enhancements exhibit a strong hemispheric asymmetry, with initially larger values in the southern hemisphere at 0900–1200 MLT followed by larger values in the northern hemisphere at 1200–1500 MLT. A forth enhancement can also be seen to develop over the Atlantic Ocean with a peak location near the 0° meridian. The dominant enhancement during these times is, however, located over the east Asian region (60°–120°E), which is about 30% larger than the Pacific enhancements. This strong enhancement of the east Asian anomaly was pointed out by Vladimer et al.  and attributed to an enhancement in the daytime equatorial vertical drift velocity over this region. With regard to the wave number four pattern, Immel et al.  suggested that it is the result of electrodynamic effects on the plasma distribution of nonmigrating tides, which are associated with latent heat release during raindrop condensation over the tropics [Hagan and Forbes, 2002, 2003]. Currently, it is not clear if other nonmigrating tidal modes and/or stationary planetary waves play a role in the generation of the strong enhancement seen in the east Asian sector during this season. Clearly, a more detailed analysis, including modeling and data analysis studies, is necessary in order to better understand the observed variations.
 During the course of the afternoon and early evening hours, the strength of the east Asian enhancement diminishes and appears to drift eastward from a peak location near 90°E at 1200–1500 MLT to a location near 110°E longitude at 1800–2100 MLT. Furthermore, the two distinct enhancements over the Pacific region (near 190°E and 260°E) that could be seen at 1200–1500 MLT are combined at later times into one extended enhancement from about 190°E to 280°E longitude. This extended enhancement is the most dominant feature in the premidnight local time sector and can be seen all through the night.
England et al. [2006a] noted that the wave number four structure is not seen during solstice periods in the UV nighttime observations. However, Figure 5b shows that during June solstice the wave number four structure is present during the afternoon but vanishes during the night when the UV observations were obtained.
Figure 5c shows the normalized TEC values during December solstice (November–February) conditions. Initially, after sunrise a broad enhancement forms over the Pacific region extending from about 190°E to 300°E longitude and a second smaller enhancement appears over east Asia from about 80°E to 110°E longitude. These two enhancements further develop until about 1500 MLT, after which the enhancement over the Pacific weakens and the enhancement over the Atlantic becomes strong and broad (300°–20°E). This Atlantic enhancement is the major feature during the late evening and premidnight hours. Note that although some evidence of a weak wave number four pattern can be seen in the afternoon hours during December solstice this structure is largely absent or washed out during this season.
Figure 6 shows an overview of the afternoon relative TEC variations for equinox (top), June solstice (middle), and December solstice (bottom) conditions. Clearly, the four separate enhancements can be seen during equinox and June solstice in both hemispheres. During December solstice conditions, however, the separate enhancements seen during the other two seasons over the east Pacific near 270°E longitude are absent, and instead, a broad enhancement is observed over the Pacific extending from 180° to 280°E longitude.
 Owing to observational constraints, most of the previous studies of the wave number four pattern have focused on the vernal equinox (March–April) and only very limited evidence exists for the development of this pattern during the autumnal equinox (September/October). Henderson et al.  used 6 d of nighttime (2130 to 2230 LT) GUVI 135.6 nm UV observations from 2 to 7 October 2002 and found that the wave number four structure was also present during these times. England et al. [2006a] used one day of OGO-4 640.0 nm observations obtained on 4 October 1967 and they also found four peaks in the nighttime emission rates. However, it is not clear if the observations during these few days are representative of the typical autumnal equinox pattern. Figure 7 shows a separation of the afternoon equinoctial data shown in the top panel of Figure 6 into vernal (March/April) and autumnal (September/October) conditions. However, in order to ensure that there is a sufficient number of data points in each bin, the MLT bins were increased to a 4-h bin from 1200 to 1600 MLT. As mentioned earlier, these data correspond to the entire 13 years of the TOPEX/Poseidon mission (August 1992 to October 2005). During both equinoctial periods, the longitudinal wave number four pattern of the TEC variations can clearly be seen in Figure 7, with the weakest enhancement over the Atlantic Ocean and the strongest enhancement over the east Asian sector. Furthermore, the locations and relative strength of the enhancements and valley regions are nearly identical during both equinoxes, indicating that the generation processes are the same during both periods. Also note that the strong similarities between the vernal and autumnal equinoxes seen in Figure 7 justified the combination of the equinoctial data into one season in Figure 5a.
3.2. Solar Cycle Variations
 There are important solar cycle variations in the overall equatorial and low-latitude TEC morphology. For example, the magnitudes of the equatorial anomalies are strongly influenced by the solar flux conditions [e.g., Jee et al., 2004] with the larger values observed during high solar flux conditions (also see Figure 4). However, a detailed analysis of the low-latitude longitudinal TEC variations has not been done. In particular, is it not clear if the wave number four structure of the afternoon and evening low-latitude ionosphere is dependent on the solar cycle conditions. Previous studies [Immel et al., 2006; Sagawa et al., 2005, England et al., 2006a] were based solely on nighttime UV observations obtained during high solar flux conditions, and Immel et al.  speculated that the troposphere-ionosphere coupling will be greatest at the peak of the solar cycle when the E region densities are highest, which would result in a less pronounced, or even absent, wave pattern during low solar flux conditions. England et al. [2006a] noted that away from solar maximum the 135.6 nm emission rates decrease rapidly, making extended observations with FUV impossible and observations with GUVI more difficult and less accurate. On the other hand, the 13-year database of TOPEX TEC observations provides the same high-quality TEC observations during low and high solar flux conditions, and consequently, is ideally suited to study solar cycle variations in the longitudinal variability of the equatorial and low-latitude ionosphere.
 In order to investigate a possible solar cycle dependence of the low-latitude longitudinal TEC pattern, we separated our normalized TEC data according to the solar flux conditions. Specifically, we created seasonal maps of the afternoon (1200–1600 MLT) and evening (1800–2100 MLT) median relative TEC variations for low (F10.7cm < 130) and high (F10.7cm > 130) solar flux conditions. The resulting maps correspond to an average F10.7cm value of 89 and 171 for the low and high solar flux periods, respectively, and, as before, represent geomagnetically moderately quiet conditions (9-h Kp average < 3.0).
 Although the absolute TEC values strongly vary with the solar flux conditions [Jee et al., 2004], Figure 8 shows surprisingly that the general longitudinal TEC morphology is largely unaffected by the solar cycle conditions. In particular, during both solar cycle conditions the wave number four pattern can clearly be observed during the afternoon and evening periods at equinox and during the afternoon period at June solstice. These results indicate that the coupling between the lower atmosphere and the ionosphere is largely independent of the solar flux conditions.
 A comparison of Figure 8 with Figure 5a also shows that the evening wave number four characteristics of the equinoctial TEC variability is better represented, as mentioned in section 3.1, when the data are separated according to their solar flux conditions. This can be understood by considering that the evening equatorial vertical drift velocities are solar flux dependent, which results in a solar cycle dependent extent of the anomaly peaks from the geomagnetic equator.
 Although the equatorial and low-latitude TEC values exhibit a clear longitudinal pattern that is fairly independent of the solar flux conditions and almost identical between the vernal and autumnal equinoxes, a significant amount of scatter remains in the TEC data that is not accounted for by the longitudinal variability. Figures 9a–9c show scatterplots of the afternoon (1400–1600 MLT) normalized TEC values versus geomagnetic latitude for equinox (Figure 9a), June solstice (Figure 9b), and December solstice (Figure 9c) conditions. In contrast to Figure 8, the MLT bin used in Figures 9a–9c was reduced from a 4-h-wide bin to a 2-h-wide bin in order to reduce the amount of scatter associated with changes in local time. The data correspond again to moderate geomagnetic activity conditions (9-h Kp average < 3.0) and low (F10.7cm < 130) and high (F10.7cm > 130) solar flux conditions. Each column shows eight scatter plots vertically stacked with each panel corresponding to a 10°-wide longitudinal bin located near the maxima and minima regions of the TEC enhancements seen in Figure 8. Superimposed are the median values and the first and ninth decile. Clearly visible in Figures 9a–9b9c are the longitudinal variations of the equatorial anomaly peaks about an average location of about ±15° geomagnetic latitude (indicated as the vertical dashed lines in Figures 9a–9b9c). Although our normalization and binning of the data has mostly eliminated variability in the data associated with variations in the solar EUV fluxes as well as variations associated with longitudinal variability, Figures 9a–9b9c show that a significant amount of scatter remains in the normalized TEC values. This scatter is largest near the poleward edges of the anomaly peaks and is of the order of about 40%. Furthermore, the scatter is largely independent of the solar flux conditions, the seasons, and the longitude of the observations. We speculate that this remaining scatter is most likely due to variability in the thermospheric neutral winds and composition, and to variability in the effects of high-latitude forcing on the low-latitude regions even during geomagnetically moderately quiet conditions. Furthermore, day-to-day variability in the tidal, planetary, and gravity wave forcing from the lower atmosphere is expected to contribute to the observed scatter. More detailed observational and modeling studies are needed to further understand the origin of this remaining variability in the equatorial and low-latitude ionosphere.
3.3. Geomagnetic Activity Variations
 During geomagnetically active conditions the equatorial and low-latitude ionosphere can be strongly affected by penetrating electric fields from high latitudes that propagate to equatorial latitudes nearly instantaneously [e.g., Senior and Blanc, 1984; Spiro et al., 1988; Fejer and Scherliess, 1997] as well as by disturbance dynamo electric fields [Blanc and Richmond, 1980; Scherliess and Fejer, 1997]. In addition, changes in the global neutral wind, composition, and temperature [Fuller-Rowell et al., 1994] can directly affect the low-latitude plasma morphology. Therefore it is not clear if the longitudinal structure of the low-latitude ionosphere, and in particular the wave number four pattern seen during quiet times would also be present during geomagnetically active times. Figure 10 shows a map of the median afternoon (1200–1600 MLT) normalized TEC values for equinox and geomagnetically active conditions. Here, we defined active times to be when the 9-h average Kp was above 4.0. Although this criterion cannot discriminate, for example, between effects associated with penetration and disturbance dynamo electric fields, it still presents a reasonable measure for the geomagnetic activity level. The average 9-h Kp value for Figure 10 is 4.9 and the average F10.7cm flux is 121. A comparison of Figure 10 and the top panel of Figure 6, which shows the corresponding map for geomagnetically quiet times, shows that the overall morphology of the longitudinal TEC variations generally follows the quiet time pattern. In particular, the wave number four structure is also seen in Figure 10, indicating that the same mechanisms modifying the quiet time plasma distribution are also effective during geomagnetically active times.
4. Summary and Conclusions
 The low-latitude ionosphere is a highly dynamic environment that exhibits significant variations with local time, altitude, latitude, longitude, solar cycle, season, and geomagnetic activity. Recently, Immel et al.  presented a study that showed a wave number four pattern in the longitudinal morphology of the nighttime airglow intensity associated with the equatorial anomaly during equinox and high solar flux conditions. They attributed this pattern to a modulation of the diurnal amplitude of atmospheric tides in the daytime E region ionosphere and a corresponding modulation of the daytime zonal electric field.
 In this study, we conducted a detailed study of the longitudinal variability of the low-latitude TEC with a special emphasis on the existence and evolution of the wave number four pattern under different geophysical conditions. In our analysis, we used more than 5 million low-latitude TOPEX TEC observations, covering the entire 13 years of TOPEX TEC data from August 1992 until October 2005. This data were used to determine the local time, seasonal, solar cycle, and geomagnetic activity dependence of the longitudinal variability of TEC at equatorial and low latitudes.
 Our study indicates that the wave number four pattern is created during the daytime hours at equinox and June solstice but is absent, or washed out by other processes, during the December solstice. During equinox, the wave number four pattern is created around noon, with well-defined longitudinal enhancements in the low-latitude TEC over east Asia, over the Pacific, and over the Atlantic Ocean. These enhancements, which are symmetric about the geomagnetic equator during this season, last for many hours and can be clearly seen past midnight. The longitudinal patterns are found to be nearly identical between the vernal (March/April) and autumnal (September/October) equinoxes and largely independent of the solar cycle conditions. The wave number four pattern is also observed during geomagnetically active conditions, indicating that the processes that create this pattern are also present during active times. The mean variations between the well-defined longitudinal maxima and minima are of the order of 20%. During the evening and nighttime hours the longitudinal pattern shifts by about 20° eastward, most likely due to an eastward drift (upward/northward electric field) of the ionospheric F region plasma. The development of this pattern during the day, as well as the observed hemispheric symmetry of the enhancements, strongly supports the interpretation of Immel et al.  that the wave number four pattern is created by a longitudinal modulation of the daytime E region dynamo electric field.
 During June solstice the wave number four pattern is also observed in the afternoon hours but, in contrast to the equinox case, it exhibits a strong hemispheric asymmetry and is not observed during the night. During this season the dominant enhancement in TEC is located over east Asia, and this enhancement is about 30% larger than the enhancements seen over the Pacific and Atlantic. The longitudinal patterns observed during June solstice are also found to be largely independent of the solar cycle conditions.
 Although the low-latitude TEC exhibits clear longitudinal variations during December solstice, with large daytime enhancements over the east Asian and Pacific regions and a third enhancement emerging in the afternoon over the Atlantic Ocean, a clear wave number four pattern is not observed during this season. This is because only one broad enhancement appears over the Pacific Ocean during this season.
 Although the equatorial and low-latitude TEC values exhibit a clear longitudinal pattern that is fairly independent of the solar flux conditions and almost identical between the vernal and autumnal equinoxes, a significant amount of scatter remains in the TEC data that is not accounted for by changes in the solar cycle or by longitudinal variability. This remaining scatter is largest near the poleward edges of the anomaly peaks and is of the order of 40%. Furthermore, the scatter is largely independent of the solar flux conditions, the seasons, and the longitude of the observations. We speculate that this scatter is most likely due to day-to-day variability in the thermospheric neutral winds and composition, as well as to variability in the high-latitude forcing of the low-latitude ionosphere even during geomagnetically moderately quiet conditions. Also, day-to day variability in the tidal, planetary, and gravity wave forcing from the lower atmosphere is expected to contribute to the observed TEC variability.
 Clearly, more observational and modeling studies are needed to better understand the variability and morphology of the equatorial and low-latitude ionosphere. In particular, future studies need to include a detailed analysis of the longitudinal variability of the daytime equatorial zonal electric field (vertical drift), which is the main driver of the equatorial and low-latitude plasma distribution. Modeling studies of the coupled lower atmosphere-ionosphere system need to include realistic descriptions of tidal activity, including migrating and non-migrating tides, to better understand the effects of this variability on the low-latitude plasma morphology. Finally, the effects of high-latitude forcing on the equatorial and low-latitude TEC variability needs to be addressed through detailed observational and modeling studies.
 The research was supported by NSF grant ATM-0533543 and Office of Naval Research (ONR) grant N0001407-0053 to Utah State University. The TOPEX data were obtained from the Physical Oceanography Distributed Active Archive Center (PO.DAAC) at the NASA Jet Propulsion Laboratory, Pasadena, California, http://podaac.jpl.nasa.gov.
 Amitava Bhattacharjee thanks Scott England and another reviewer for their assistance in evaluating this paper.