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 After sunset on 8 November 2004, the ionospheric total electron content (TEC) was observed to have anomalously increased following a severe daytime positive TEC storm at longitudes of Japan. The observation was made using a dense GPS receiver network, and covered a geographic latitudinal range of 27 to 45°N. There was a greater increase in TEC at higher latitudes in the evening, and the TEC reached 90 TEC units at 45°N (∼40°N magnetic latitude) at 1145 UT (2045 LT). The TEC enhancement exhibited features significantly different from those of positive TEC storms normally observed at Japan's longitudes. These features are interpreted as low-latitude signatures of a storm enhanced density (SED). Previously, SEDs were reported only at longitudes of America, and this led to the hypothesis that geomagnetic field configurations at these longitudes play a role in their formation. The present observations indicate that SEDs can be observed at other longitudes.
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 Associated with geomagnetic storms, the ionospheric total electron content (TEC) often significantly increases. The important mechanism of TEC enhancement is uplift of the ionosphere into higher altitudes with a small recombination rate during sunlit hours when photo-ionization occurs [Tsurutani et al., 2004; Maruyama et al., 2004, Mannucci et al., 2005]. Several processes possibly significantly raise the ionospheric height, including E × B drifts caused by eastward directed prompt penetrating (PP) magnetospheric electric fields [Spiro et al., 1988] and drags by equatorward neutral winds [Fuller-Rowell et al., 1994].
 Another positive TEC disturbance is the dusk effect localized near the 18 LT at midlatitudes [Buonsanto, 1995]. This is associated with the higher-latitude storm enhanced density (SED) spanning a wide extent of local time in the afternoon sector [Foster, 1993]. The advection of low latitude dense plasma due to the E × B drift by the eastward and poleward electric field is thought to be the mechanism of SED and dusk effect TEC enhancement [Foster, 1993; Foster and Rideout, 2005]. Previously, SEDs were reported only at longitudes of America, and this led to the hypothesis that geomagnetic field configurations at those longitudes play a role in their formation [Foster, 1993]. A better understanding of the formation conditions, including electric field disturbances, will be achieved if the SED phenomena in other longitude sectors is investigated.
 We analyzed a chain of ionospheric disturbances observed at Western Pacific longitudes on 8 November 2004 that was associated with a great geomagnetic storm, and then connected the evening part of the TEC disturbances to an SED.
2. Data Sets Used
 The area covered by the GPS Earth Observation Network (GEONET), a dense GPS receiver network over Japan, was divided into 32 cells, 2 × 2° in longitude and latitude, as shown in Figure 1. From about 1200 GEONET receivers, we chose 330 homogeneously distributed receivers for the TEC calculations. The vertically converted TEC at a given time and in a given cell was assumed to be constant, and the values were determined every quarter hour. The details of the algorithm used are described elsewhere [Ma and Maruyama, 2003]. Quarter hourly ionograms were obtained from four ionosonde stations, Wakkanai (45.4°N, 141.7°E; 40.5°N magnetic latitude), Kokubunji (35.7°N, 139.5°E; 30.0°N magnetic latitude), Yamagawa (31.2°N, 130.6°E; 26.1°N magnetic latitude), and Okinawa (26.7°N, 128.2°E; 21.2°N magnetic latitude), as shown in Figure 1. The ionospheric peak height (hmF2) was derived from M3000F2 [see Maruyama et al., 2004]. The DMSP-F15 satellite made a direct pass over Japan when the severe TEC enhancement was observed in the evening. Total ion density and plasma drift data obtained by the satellite were used for interpreting the TEC data.
3. Geomagnetic Conditions
 An intense ionospheric storm occurred on 7 November 2004 in the declining phase of solar activity. The first panel of Figure 2 shows IMF Bz as observed by the ACE spacecraft. The second panel shows the AE index (the quick-look AE index supplied by World Data Center for Geomagnetism, Kyoto University). The third panel shows the Dst index. The Dst started to decrease at 2100 UT on 7 November when the IMF Bz turned southward and reached its maximum depression of −373 nT at 0600 UT. The vertical dotted line in the storm recovery phase indicates the time when the anomalous evening TEC enhancement was observed.
 Contour maps of the TEC variations as a function of time and latitude are shown in Figure 3. The F1-region (200 km) sunrise and sunset times are indicated by dotted lines and local noon (135°E mean local time) is centered. Figure 3a shows the TEC variations from 7 to 8 November. The numbers on the contours are TEC units of 1 × 1016 m−2. The difference in TEC between the storm and quiet days (ΔTEC) is shown in Figure 3b. This depiction shows that ΔTEC gradually started to increase in the morning, and the enhancement was largely reinforced three times until midnight. The first peak of TEC occurred at 0500–0600 UT at all latitudes with a slight delay of the peak time at higher latitudes. The second one occurred at 0800–1000 UT and was confined to latitudes lower than 37°N near sunset. The last one occurred at around 1200 UT and was distinct at latitudes higher than 35°N.
 Large TEC enhancements are generally associated with a layer uplift [Tsurutani et al., 2004; Maruyama et al., 2004; Mannucci et al., 2005]. Thus, we plot the hmF2 variation at the four stations from north (top) to south (bottom) in Figure 4. At about sunrise (inverted open triangles), the layer was raised and the uplift continued beyond the sunset (inverted closed triangles). The layer uplift was responsible for the gradual TEC enhancement after sunrise. Relatively small amounts of TEC enhancement in the morning might be due to a large solar zenith angle. The start of the uplift at Wakkanai was at 1900 UT and that at Okinawa was at 2100 UT as indicated by the vertical bars. Thus, the ionospheric disturbances started near sunrise were most probably a traveling atmospheric disturbance (TAD) that originated at higher latitudes [Prölss, 1993].
 About the prominent TEC enhancement at 0500–0600 UT, the wide latitudinal extent and the delay of the peak at higher latitudes resemble the feature of the previously analyzed TEC storm on 6 November 2001 [Maruyama et al., 2004]. That event was interpreted as an effect of prompt penetrating electric field [Tsurutani et al., 2004]. The time of the peak enhancement was delayed by 3 to 4 hours (longer delay at higher latitudes) after the simultaneous uplift of the layer. Similar delay of TEC enhancement after the start of the layer uplift is reported by Mannucci et al. . In the present event, however, a simultaneous increase in hmF2 at the four stations, which is a signature of prompt penetrating electric fields, was not recognized before the TEC enhancement. We need further studies on the ionospheric disturbances in this period.
 The broad peak of layer uplift seen at Okinawa between 0600 and 0900 UT (hatched area in Figure 4) clearly corresponded to the TEC enhancement that was confined to the lower latitudes and peaked at around 0900 UT. A similar but small layer uplift was observed at Yamagawa too. There was a time delay of 3 hours between the start of the uplift at Okinawa and the peak of the TEC enhancement.
 Finally, regarding the prominent TEC enhancement at around 1200 UT, we note a TID signature, delayed uplift at lower latitudes as indicated by the dashed line in Figure 4. By 1115 UT the layer height returned to the quiet level at Wakkanai, after which the TEC was largely enhanced. As the layer uplift occurred at midlatitudes in the dusk, the mechanism of TEC enhancement should be different from the previous two TEC events.
 Although Figure 3 depicts the latitudinal-local time perspective on the disturbance, the longitudinal variations were averaged over cells aligned on an east-west line at each latitude. Figure 5 shows the TECs for the cells selected at specific longitude or latitude. Figure 5a shows the TECs for four of the cells aligned in a north-south direction at 141°E (Nos. 22, 23, 24, and 25 in Figure 1). At 43°N (No. 25; 37.9°N magnetic latitude), the TEC started to increase after sunset and reached a peak at 1130 UT, which was larger than the daytime maximum at the same location. The increase was about 75 TEC units. The start and peak times of the enhancement were delayed by 45 min from 43° to 37°N (No. 22; 31.2°N magnetic latitude), and the increase was lower at lower latitudes; it was 20 TEC units at 37°N. The observed peak of the disturbance appeared to move south at a rate of 8°/hr.
Figure 5b shows the TECs for the three cells aligned in an east-west direction at 41°N (Nos. 19, 24, and 28). After sunset, TEC enhancement began and peaked earlier in the east (No. 28). The observed peak of the disturbance appeared to move west at a rate of 8°/hr or half the speed of the sunset terminator.
 The equatorward movement of the TEC disturbances invokes a connection with traveling atmospheric disturbances (TADs) that originate in the auroral region [Buonsanto, 1995]. The layer height variation at around 0900 UT at Wakkanai and Kokubunji (Figure 4) exhibited a TAD activity. Immediately after this, the TEC increased by 53 TEC units in one hour for cell 25, which corresponded to a flux of 1014 m−2s−1. At midlatitudes at L ∼ 1.5, plasma densities at the conjugate ionosphere have been shown to be directly coupled through the field aligned interhemispheric H+ flux [Bailey et al., 1987]. If the TEC enhancement was caused by an interhemispheric or protonospheric flux in conjunction with an equatorward neutral wind of the TAD, which supported the ionosphere at high altitudes, the observed flux would be larger than the limiting flux of topside H+ field aligned flow by two orders [Richards and Torr, 1985]. Thus, the TEC increase should be caused by a horizontal advection of dense plasma.
Figure 6a shows the total ion density at the height of about 850 km observed by DMSP-F15. The solid line is for 8 November (the satellite track of which is shown in Figure 1) and the dashed line is for a similar orbit on 6 November for a quiet-time reference. The ion density on 8 November was elevated over a wide latitude range and exhibited a density peak at (41.1°N, 143.4°E) at 1100 UT. A contribution of the electron density to the TECs is the largest at the F layer peak, and we adopted a thin layer approximation to relate TECs with geographic locations, in which the reference height was taken at 400 km [Ma and Maruyama, 2003]. If the TEC enhancement was caused by the advection of dense plasma due to the E × B drift, the density enhanced region should exhibit field-aligned structure. The point at which the density peak was observed by DMSP at 850 km can be mapped along the magnetic field line down to 400 km at (43.5°N, 143.1°E), which is in cell 29. The TECs at 1100 UT for cells 27, 28, 29, and 30 were 54.7, 74.2, 95.7, and 85.2 TEC units, respectively. Thus the density peak observed by DMSP along the satellite track is consistent with the TEC observations on the ground. At lower latitudes in the figure, another density peak corresponding to the northern part of the equatorial anomaly crests, evidence of enhanced equatorial fountain, was clearly identified near 19°N (12°N magnetic latitude) on 8 November, but not on 6 November.
Figure 6b shows the westward ion drift velocity obtained by the same satellite (cross-track y component). The velocity at the density enhancement, ∼250 m/s, was slightly faster than the apparent westward movement of the TEC frontal structure at a rate of 8°/hr or 200 m/s. However, the ion drift velocities along the spacecraft's path and vertical direction (cross-track z component) indicated a northward field-perpendicular velocity of ∼40 m/s. Thus, the apparent southward movement of the TEC frontal structure was not a true plasma movement, but the density-enhanced structure was formed by a plasma stream from east-southeast to west-northwest. Such TEC-enhanced structure, slanting poleward as they move from east to west in an Earth-fixed coordinate system with north at the top, during periods of magnetic disturbances strongly invokes the SED. Further poleward, DMSP observed strong intensification of the subauroral polarization stream, which also signifies an SED/TEC plume event [Foster et al., 2002; Foster and Rideout, 2005].
 In previous studies [Foster, 1993; Vlasov et al., 2003; Foster and Rideout, 2005], SED/TEC plumes are associated with a positive storm at lower latitudes, which can be distinguished from the density enhancement of SED at higher midlatitudes with a trough between the two regions. However, the two regions of density enhancement are connected to each other such that the lower latitude dense plasma is a source of the SED/TEC plumes [Foster, 1993]. In this regard, the current event also exhibited a severe daytime TEC storm.
 The 250-m/s westward plasma drift in the TEC-enhanced region is less than that of the sunset terminator movement (400 m/s at 41°N). This means that the dense plasma originated in the east (and south) at early local times. In addition to the general daytime positive storm, we noted a further TEC enhancement at lower latitudes at around 0900 UT in Figure 3. This corresponds to the layer uplift seen between 0600 and 0900 UT (1500 and 1800 LT) in Figure 4, and is interpreted as a poleward expansion of the equatorial anomaly. Plasma accumulation in the equatorial anomaly crest region due to an enhanced plasma fountain is the consequence of the plasma drift and diffusion acting over time. The electron density is expected to have maximized somewhat after the upward E × B drift subsided [Lin et al., 2005]. At around 0700 UT, the layer reached its maximum height and the upward E × B drift ceased. The peak of TEC was observed two hours later in Figure 3, which is consistent with the modeling results of Lin et al. . Although the origin of the disturbance is not very clear, the additional poleward expansion of the equatorial anomaly following the daytime severe TEC storm could promote the formation of the SED.
 Another key point in the formation of SEDs is the sustained driving mechanism of the westward and poleward plasma advection at lower midlatitudes. We noted that the ionospheric height was consistently high during the day, evidence of enhanced equatorward circulation. Such thermospheric circulation disturbances generate a disturbance dynamo electric field. Model calculations by Blanc and Richmond  revealed a poleward disturbance electric field of about 5 mV/m throughout the day and eastward disturbance electric field of 1 to 3 mV/m at night after 19 hr LT at midlatitudes (44°), which could be the driving force of the plasma drift.
 We observed an anomalous TEC enhancement at latitudes higher than 35°N at about 21 LT at Western Pacific longitudes during a magnetic disturbance. The TEC-enhanced region appeared to be aligned in a northwest to southeast direction and appeared to move in a westward component at a rate of 8°/hr. Analysis of DMSP ion drift data suggested that a plasma stream toward the west-northwest formed the spatial structure of anomalous TEC enhanced region, which was interpreted as a low-latitude signature of SED. A narrow TEC-enhanced region passed over northern Japan that displayed a poleward gradient in a time-latitude TEC map. Previously reported SEDs at longitudes of America led to the hypothesis that the equatorward offset of the geomagnetic pole at those longitudes and/or the Brazilian magnetic field anomaly play a role in SED formation. At Japan's longitudes, the magnetic equator shifts to the north, and the equatorial geomagnetic field strength is larger than at any other longitude. The present observations show an SED in the Japan sector and suggest that SEDs can be observed at any longitude.
 The author gratefully acknowledges the Center for Space Sciences at the University of Texas at Dallas and the US Air Force for providing the DMSP thermal plasma data.