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PUBLICATIONS Journal of Geophysical Research: Space Physics RESEARCH ARTICLE 10.1002/2016JA023029 Key Points: • The longitudinal variations of topside Ne have similar patterns to hmF2, except in the northern winter (DS) and southern winter (JS) • In the southern winter (JS) the topside Ne has similar patterns to NmF2, whereas hmF2 does not change much with longitude • The patterns of topside Ne in winter are different from other seasons in the Northern Hemisphere

Correspondence to: F. Su, [email protected]

Citation: Su, F., W. Wang, A. G. Burns, X. Yue, F. Zhu, and J. Lin (2016), Statistical behavior of the longitudinal variations of daytime electron density in the topside ionosphere at middle latitudes, J. Geophys. Res. Space Physics, 121, 11,560–11,573, doi:10.1002/ 2016JA023029. Received 9 JUN 2016 Accepted 9 NOV 2016 Accepted article online 11 NOV 2016 Published online 28 NOV 2016

Statistical behavior of the longitudinal variations of daytime electron density in the topside ionosphere at middle latitudes Fanfan Su1,2, Wenbin Wang2, Alan G. Burns2, Xinan Yue3, Fuying Zhu1, and Jian Lin1 1

Key Laboratory of Earthquake Geodesy, Institute of Seismology, China Earthquake Administration, Wuhan, China, 2High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado, USA, 3Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

Abstract Electron density in the topside ionosphere has significant variations with latitude, longitude, altitude, local time, season, and solar cycle. This paper focuses on the global and seasonal features of longitudinal structures of daytime topside electron density (Ne) at middle latitudes and their possible causes. We used in situ Ne measured by DEMETER and F2 layer peak height (hmF2) and peak density (NmF2) from COSMIC. The longitudinal variations of the daytime topside Ne show a wave number 2-type structure in the Northern Hemisphere, whereas those in the Southern Hemisphere are dominated by a wave number 1 structure and are much larger than those in the Northern Hemisphere. The patterns around December solstice (DS) in the Northern Hemisphere (winter) are different from other seasons, whereas the patterns in the Southern Hemisphere are similar in each season. Around March equinox (ME), June solstice (JS), and September equinox (SE) in the Northern Hemisphere and around ME, SE, and DS in the Southern Hemisphere, the longitudinal variations of topside Ne have similar patterns to hmF2. Around JS in the Southern Hemisphere (winter), the topside Ne has similar patterns to NmF2 and hmF2 does not change much with longitude. Thus, the topside variations may be explained intuitively in terms of hmF2 and NmF2. This approach works reasonably well in most of the situations except in the northern winter in the topside not too far from the F2 peak. In this sense, understanding variations in hmF2 and NmF2 becomes an important and relevant subject for this topside ionospheric study. 1. Introduction The topside ionosphere is a highly dynamic region that varies significantly with latitude, longitude, altitude, local time, season, and solar cycle. These variations have been explored for decades with many ground-based and satellite instruments. One interesting variation is the wave-like longitudinal patterns in the ionosphere around the equator and at low latitudes at a fixed local time. This has become a topic of much interest in the years since Sagawa et al. [2005] and Immel et al. [2006] reported this wave number 4 longitudinal structure. The vertically propagating nonmigrating tides of tropospheric origin are believed to be the primary mechanism driving this variation [e.g., Sagawa et al., 2005; Immel et al., 2006; Scherliess et al., 2008; Wan et al., 2008; England et al., 2010, and references therein].

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In the ionosphere, near the F2 layer peak at middle latitudes, the longitudinal variations of the electron density have been studied for decades. Millward et al. [1996] proposed a mechanism for these variations that involves the large offset between the geomagnetic axis and the Earth’s spin axis in the Southern Hemisphere. Zou et al. [2000] studied F2 layer electron density from ionosonde data at middle latitudes and then reproduced their longitudinal variations using the Coupled Thermosphere-Ionosphere-Plasmasphere model. These simulated results showed that the noon ionospheric F2 peak density (NmF2) in the eastern longitudes is greater than that in the western longitudes at middle latitudes in all seasons. They explained these observations in terms of the differences between the geographic and geomagnetic poles and the effect of solar zenith angle [Zou et al., 2000; Rishbeth et al., 2000]. Liu et al. [2010] investigated CHAMP measurements at 400 km and found daily maxima of electron density (Ne) at midnight in some longitudinal regions at middle latitudes. They believed that neutral winds combined with the geomagnetic field configuration were the key cause of this behavior. Liu et al. [2011] reported the longitudinal patterns of NmF2, F2 peak height (hmF2), and Chapman scale height in 2008–2009 using COSMIC data and also attributed the summer nighttime enhancement in NmF2 and increase in hmF2 to thermospheric wind effects that changed with geomagnetic field configuration and solar LONGITUDINAL VARIATIONS OF TOPSIDE NE

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photoionization. Zhang et al. [2012] studied the east-west differences in the electron density over the continental United States using data from the incoherent scatter radar at Millstone Hill (42.5°N, 288.6°E). They explained these differences in terms of the combined effect of geomagnetic declination and zonal winds and brought out the possibility of a wave 2 longitudinal structure of electron density in the Northern Hemisphere. The east-west difference in the F layer electron density in China displayed a diurnal variation that was similar but smoother than the one over North America [Zhao et al., 2013]. Xiong and Lühr [2014] used tidal signatures to describe the middle-latitude summer night anomaly of electron density from CHAMP and Gravity Recovery and Climate Experiment satellites, which had a wave 1 longitudinal structure in the Southern Hemisphere and a wave 2 longitudinal pattern in the Northern Hemisphere. Wang et al. [2015] employed the global ionosphere-thermosphere model and Ne measurements from the CHAMP satellite within 131 days centering at SE in 2004 to analyze the relative importance of the possible physical drivers of the longitudinal changes of Ne at middle latitudes, including the geomagnetic field geometry, neutral winds, solar illumination, migrating tides, and enhanced geomagnetic activities at high latitudes. Nevertheless, the mechanisms that drive the longitudinal changes in the F region ionosphere are still a controversial issue partly due to the limitations of observation techniques and a lack of sufficient data, such as neutral winds and composition. In the topside ionosphere one or two scale heights above the F2 layer peak, longitudinal variations at middle latitudes have been less studied than those at low latitudes, partially because the electron density around the equator is much higher than that at middle latitudes, which leads to significant equatorial effects in the global plots of ionospheric total electron content (TEC) and electron density (Ne) and the structure at middle latitudes would be not visible. Sultan and Rich [2001] estimated the longitudinal variations of meridional neutral winds around 20° from the asymmetry observed in latitude profiles of the topside ion density from Defense Meteorological Satellite Program at 840 km. Kakinami et al. [2011] used DEMETER satellite data to create a global map of the electron density in the topside ionosphere and discussed in detail the longitudinal variations around the equator. Pedatella et al. [2011] showed the seasonal and local time variability of the longitudinal structures in ionospheric TEC at low and middle latitudes above 800 km with COSMIC data and analyzed the TEC wave features in longitude between 10° magnetic latitudes. Thus, most research on the longitudinal variations of the topside ionosphere has focused on changes at low latitudes. Some researchers have used TEC to investigate longitudinal variations at middle latitudes in the ionosphere. Jee et al. [2004] used almost 10 years of TOPEX TEC data to study longitudinal variations of TEC and found that the longitudinal variations of TEC at 45° magnetic latitude closely follow the longitudinal variations of magnetic declination in the Southern Hemisphere. Jee et al. [2009] displayed global longitudinal variations of TOPEX TEC maps at middle and high latitudes for a given local time in the Southern Hemisphere and considered the role of neutral winds in producing these patterns. Zhang et al. [2011] used the ground-based GPS TEC to study the east-west coast differences over the continental U.S. and found that TEC was substantially larger on the east coast than on the west coast in the evening, whereas the opposite effect occurred in the morning. They proposed that these longitudinal differences are caused by the combined effect of the magnetic declination and thermospheric zonal winds. Therearenighttimeplasmadensityenhancements,includingtheWeddellSeaAnomaly,whicharecharacterized byagreaterelectrondensityatnighttimethanduringthedaytime[e.g.,Penndorf,1965;Horvath,2006;Burnsetal., 2011; Ren et al., 2012; Zhang et al., 2013; Slominska et al., 2014, and references therein]. These phenomena only appear in a limited area. In this paper we focus on the longitudinal structure of the topside Ne more thanone scale height above the F2 layer peak at middle latitudes during the daytime, which attract much less attention than nighttime phenomena. In general, the same explanations have been invoked to explain the longitudinal variations,withoutconsideringwhethertheyareprimarilyF2 peakeffectsortopsideones.Infact,thelongitudinal variations in different ionospheric regions (F2 peak, topside) may be caused by different physical processes. The F2 layer peaks at solar minimum are lower than those at solar maximum. Satellite observations made at a fixed height in the topside in solar minimum will be farther away from the F2 layer peak than in solar minimum, and this may alter the interpretation of the results. Moreover, the topside ionosphere and F2 layer peak are linked in a complicated way. Thus, a study on longitudinal structure of the topside Ne is helpful for exploring the physical processes that drive the topside ionosphere and the factors that affect our interpretation of this region. To examine the causes of longitudinal structure, we have combined two databases at solar minimum in 2007–2009: one from DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake

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Regions) and the other from COSMIC (FORMOSAT-3/Constellation Observing System for Meteorology, Ionosphere, and Climate—COSMIC). Measurements from the DEMETER satellite have good spatial coverage and resolution at an orbital altitude of ~670 km. A global view of longitudinal variations of the electron density can be obtained with these observations. The high spatial resolution of DEMETER measurements makes it possible to observe the latitudinal and seasonal variations of longitudinal structures. These data were collected in the last solar minimum, when the F2 layer peak was much lower than the orbit altitude. COSMIC measurements in 2007–2009 showed that the F2 layer peak heights at middle latitudes were mostly around 200–230 km, and the plasma scale heights around 670 km at middle latitudes were mostly around 280– 340 km from COSMIC electron density profiles [Schunk and Nagy, 2009]. The distances between the F2 layer peak and the orbit altitude around 670 km were much larger than one scale height. The physical processes in the topside may be different from those in the F2 layer peak. Here we investigate the longitudinal variations of Ne at middle latitudes in the topside ionosphere and in the F2 region to understand how the observed variations differ in the two regions. We use a normalization method to decrease the influence of the significant latitudinal change of Ne on data analysis. The data from DEMETER and COSMIC were analyzed in a similar way to explore some of the possible processes that might cause these longitudinal variations in the topside ionosphere. These data are described in section 2. The seasonal and global distributions of the geographic longitudinal variations for topside Ne are presented in section 3. In section 4 we showed the longitudinal variations of NmF2 and hmF2 and analyzed the connections between topside Ne and the parameters at the F2 peak. In section 5 we discuss the possible causes of longitudinal structures of topside Ne at middle latitudes. The findings of this study are summarized in section 6.

2. Data Description In this study, we use DEMETER satellite measurements made at middle latitudes. DEMETER was a French microsatellite launched in June 2004 by Centre National d’Etudes Spatiales (CNES) and ended its scientific mission in December 2010. Its measurements were widely used to explore the characteristics of the topside ionosphere [Wang et al., 2010; Kakinami et al., 2011; Piddyachiy et al., 2011; Parrot and Berthelier, 2012; Slominska et al., 2014]. It was launched to a circular orbital altitude of 710 km, and then lowered to 680 km in December 2005, with a descending node at ~10:30 LT during the daytime and an ascending node at ~22:30 LT in the night sector. The orbital period was around 100 min, and the satellite observations nearly continuously spanned the regions between 65° and +65° of invariant latitudes. A detailed description of the instruments can be found in the Planetary and Space Science DEMETER Special Issue in 2006 [Lebreton et al., 2006]. COSMIC consists of six microsatellites, which were launched in April 2006, and is operated by Taiwan’s National Space Organization, with the science data products generated by the University Corporation for Atmospheric Research [Schreiner et al., 2007]. The application of these COSMIC satellites to ionospheric studies was described in detail by Lei et al. [2007] and Yue et al. [2014]. In this paper, we use the Electron Density Profiles (EDPs), which are one of the space weather data products of COSMIC. These EDPs were derived from Global Navigation Satellite Systems occultation data rather than direct measurements [Yue et al., 2012]. The global distribution of topside Ne, the electron density of F2 layer peak (NmF2), and the height of F2 layer peak (hmF2) can be obtained from these EDPs. DEMETER had more than 9.8 million in situ Ne measurements around 10:30 LT in 2007–2009. These data ensure a high spatial resolution description of the longitudinal characteristics in topside Ne. Due to higher spatial resolution of DEMETER measurements and the consistency of longitudinal variations between COSMIC Ne at ~670 km and DEMETER in situ Ne (as described in the next section), we use DEMETER measurements to display the seasonal and spatial distribution of Ne longitudinal variations in the topside ionosphere and use NmF2 and hmF2 from COSMIC to assist in the interpretation of these longitudinal variations in the topside ionosphere. Although DEMETER Ne at 670 km had similar longitudinal patterns to COSMIC Ne around 670 km, they have different absolute values. The relative variations of Ne measurements from DEMETER have been validated in many studies [e.g., Wang et al., 2010; Piddyachiy et al., 2011; Parrot and Berthelier, 2012; Kakinami et al., 2013; Slominska et al., 2014; Su et al., 2015], but the absolute values are not usable for comparison with the presumably more accurate COSMIC products.

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We studied the global characteristics of the geographical longitudinal variations of topside Ne within 30 days around the equinoxes and solstices in 2007–2009, when all the COSMIC satellites were at an altitude of ~800 km and were evenly distributed in longitude. The yearly mean values of the F10.7 index in 2007–2009 were 73.1, 69.0, and 70.6. Our study is also for low geomagnetic activity. The data during the 3 h interval with Kp > 3 and the next 24 h were disregarded. The occurrences of geomagnetic storms were less common, and the geomagnetic activity was very low during the solar minimum years of 2007–2009. The estimation of median value in each bin also could reduce the effects of geomagnetic storms. Thus, the effects of geomagnetic storm on our results are negligible. Considering the fact that the geographical longitudinal distributions of topside electron density in low geomagnetic activity periods are similar from 2007 to 2009, the electron densities from the DEMETER at ~10:30 LT in the same bin in different years were collected together and median values were estimated for further processing. We use the following formula to calculate the normalized electron densities to extract geographical longitude variations at different geomagnetic latitudes, Nelon ¼

Nei  Nem Nem

(1)

where Nelon is the geographic longitudinal variation of Ne at the same magnetic latitude, Nei is the median of Ne in each bin, and Nem is the mean value of Ne from the bins with the same magnetic latitude. With the normalization to the mean value at each latitudinal bin, we could compare the longitudinal variations at different latitudes without the influence of electron density changes with latitude.

3. The Statistical Characteristics of the Longitudinal Variations Figure 1 shows the daytime global distribution of geographical longitudinal variations of Ne in the topside ionosphere, which is normalized to the mean value at the same geomagnetic latitude and denoted as Nelon in equation (1) in section 2, at about 670 km around 10:30 LT in 2007–2009 using DEMETER measurements. The measurements are within 30 days around March equinox (ME), June solstice (JS), September equinox (SE), and December solstice (DS). The white areas correspond to locations where the instrument was shut off [Lebreton et al., 2006]. In Figure 1 the locations of the peaks of longitudinal variations around the geomagnetic equator are different in the four seasons. These wave-like longitudinal patterns around the geomagnetic equator have been the subject of many studies [e.g., Walker, 1981; Su et al., 1996; Sagawa et al., 2005; Immel et al., 2006; Scherliess et al., 2008; Wan et al., 2008; England et al., 2010]. From Figure 1 we can see that the longitudinal variations at middle latitudes in the Southern Hemisphere and in the Northern Hemisphere are asymmetric. There is a large peak in the eastern hemisphere in the Southern Hemisphere during all seasons, whereas there are two weak peaks in the Northern Hemisphere. As we are mainly interested in the longitudinal patterns at geomagnetic middle latitudes, we study these patterns in several latitude bands with a latitudinal resolution of 2° and a longitudinal resolution of 5°. Figure 2 shows the normalized geographical longitudinal variations of Ne at different geomagnetic latitudinal bands in each season. The maximum longitudinal variation of topside Ne in the Northern Hemisphere is about 0.3, whereas in the Southern Hemisphere the maximum variation is about 0.6, which is much larger than that in the Northern Hemisphere. At geomagnetic latitudes from 36°N to 52°N, there are two main peaks. The two positive peaks in ME, JS, and SE have roughly the same magnitude and are located near ~130°W and 75°E longitudes. For the negative peaks, the magnitude of the one near 40°W longitude is larger than the one at ~150°E longitude. The peak magnitudes do not change greatly with magnetic latitude as shown in Figure 2. The pattern in the Southern Hemisphere is totally different from that in the Northern Hemisphere. At geomagnetic latitudes between 36°S and 52°S, there is one main positive peak located in the eastern hemisphere, and the magnitude becomes greater as the geomagnetic latitude increases. The main negative peak, however, does not appear to have large magnetic latitude dependence. There is a slight phase shift in the southern plots as the zero crossing moves from about 30°W to 30°E as latitudes become more southward. These gradual changes of geographic longitudinal variations with geomagnetic latitude are significant in most seasons except in southern summer (DS), which indicates that the sources for geographic longitudinal variations might also have a gradual latitudinal variation.

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Figure 1. The global distribution of the normalized geographic longitudinal variations of Ne, which is normalized to the mean value at the same geomagnetic latitude band, at about 670 km with DEMETER daytime measurements in 2007–2009 around March equinox (ME), June solstice (JS), September equinox (SE), and December solstice (DS).

In the Northern Hemisphere, the zero crossing of variations coincides with the sign changes of magnetic declination in most seasons except in northern winter (DS). Zhang et al. [2012] analyzed the electron density at 350, 400, and 450 km from the incoherent scatter radar at Millstone Hill (42.5°N, 288.6°E) and thermospheric zonal winds from the Fabry-Perot interferometer at the same location and proposed the geomagnetic declination-zonal wind mechanism for the longitudinal variation in ionospheric electron density. In the Southern Hemisphere, the pattern of longitudinal variations of topside Ne is consistent with the variation of magnetic declination, but the locations of the zero crossing of Ne variation are different from the zero magnetic declination for all seasons. The patterns around DS in the Northern Hemisphere (northern winter) are different from the other seasons. The eastern peaks of longitudinal variations in northern winter are lower than those in others season. The longitudinal patterns in the Southern Hemisphere appear not changing with season, but the locations of the peaks change with seasons. The longitudinal variations at different geographic latitudinal bands have similar patterns with slight phase shifts and gradually changing magnitudes. We checked this result in each individual year with the DEMETER data from 2006 to 2009 and the COSMIC data from 2007 to 2010, and similar results are obtained. We tried to plot the results in both geographic and geomagnetic coordinates and found that whatever coordinates were selected, the “unusual” behavior of topside Ne in the northern winter and hmF2 in the southern winter still occur. The behavior in winter needs more research. When we compared the geographic longitudinal variations of the topside electron density from DEMETER in the daytime with the results of the IRI 2012 model [Bilitza et al., 2014] at the same altitude and geomagnetic

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Figure 2. The normalized geographic longitudinal variations of Ne at about 670 km with DEMETER daytime measurements in 2007–2009. (left column) Northern Hemisphere and (right column) Southern Hemisphere. From top to bottom for each row: ME, JS, SE, and DS. Notice that the scales of the Y axis in Figure 2 (left column) are different from those in Figure 2 (right column).

latitude, we found that the IRI 2012 model could sometimes reproduce the density peaks, but the locations and the magnitudes of the peaks were not consistent with the measurements.

4. Physical Interpretation We use the longitudinal variations of NmF2 and hmF2 from COSMIC to assist in the interpretation of the Ne longitudinal variations in the topside ionosphere. Burns et al. [2012] reported the seasonal, semiannual, and annual features of NmF2 and hmF2 with COSMIC data. In Figure 3 we show the global maps of the normalized geographic longitudinal variations of NmF2 calculated from COSMIC EDPs products. The data were collected from 9 LT to 12 LT within 30 days around ME, JS, SE, and DS in 2007–2009, with a latitudinal resolution of 2° and a longitudinal resolution of 10°. Although the inversion procedure to derive the COSMIC EDPs assumes that the ionosphere has spherical symmetry [Yue et al., 2012], both NmF2 and Ne around 670 km demonstrate hemispheric asymmetries. The patterns of NmF2 longitudinal variations at middle latitudes are similar to the topside Ne in Figure 1. We assume that it is due to the mapping of F2 layer electron density along the geomagnetic field lines to the topside ionosphere. The longitudinal variations of Ne at the F2 layer peak are less significant than those in the topside except around JS in the Southern Hemisphere. Figure 4 provides line plots of the geographic longitudinal variations of NmF2 at different geomagnetic latitudes in each season. We can see the twin-peaked patterns in the Northern Hemisphere except during the DS (northern winter) and weak one-peaked patterns in the Southern Hemisphere except around ME (southern autumn). In the northern winter (DS) and southern winter (JS), the variations are more significant than other seasons.

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Figure 3. Same as Figure 1 but for NmF2 from COSMIC.

The global map of the normalized geographic longitudinal variations of hmF2 was also calculated from COSMIC EDPs products and shown in Figure 5. The patterns are mostly similar to the in situ Ne of DEMETER at ~670 km. Figure 6 shows the line plots of geographic longitudinal variations of hmF2 at different geomagnetic latitudes in each season. The two-wave patterns in the Northern Hemisphere are recognizable except around DS (northern winter). The one-wave patterns in the Southern Hemisphere occur except during the JS (southern winter) case. The hmF2 in both northern and southern winter does not change much at different latitudes, and the most significant variations with geographic longitudes appear in northern and southern summer. Given the nature of the topside ionosphere, it is reasonable to assume that there should be a relationship between these F2 layer peak parameters and the topside Ne. DEMETER in situ Ne data measured at 670 km in conjunction with COSMIC observations of NmF2 and hmF2 have been used here to study how the zonal distribution of the density and height at the F2 layer peak relates to zonal variations of Ne in the topside. Global maps of geographic longitudinal variations of Ne, NmF2, and hmF2, which are shown in Figures 1, 3, and 5, display the main features of these variations. When we compare these figures, we see that the patterns of the in situ topside Ne at middle latitudes are similar to those of both hmF2 and NmF2. In the Northern Hemisphere, the peaks of in situ Ne are located in a similar longitudinal range to hmF2. NmF2 has twin-peaked patterns during the ME and JS, and one single peak during the DS, but their longitudinal locations are not consistent with those of the in situ Ne, and there is no clear pattern during the SE in Figure 4. In the Southern Hemisphere, there is a peak in the topside Ne in the eastern hemisphere in all four seasons, although the longitude of the peak changes with season. Similarly, there is a topside Ne trough in the western hemisphere in all four seasons. The locations of density trough also change with season.

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Figure 4. Same as Figure 2 but for NmF2 from COSMIC. Notice that the scales of the Y axis in Figure 4 (left column) are different from those in Figure 4 (right column).

These peaks roughly correspond to the longitudinal changes in hmF2 and NmF2 in the Southern Hemisphere as well. For hmF2 the exception occurs in the JS in the Southern Hemisphere, when hmF2 does not have an obvious peak. NmF2 behaves like topside Ne during the JS but does not have obvious peaks in other seasons. Both NmF2 and hmF2 can influence the topside Ne. A possible physical mechanism that might be connected to these longitudinal variations is illustrated schematically in Figure 7. The yellow line denotes an electron density profile with larger NmF2 and hmF2, and the green line denotes an electron density profile with smaller NmF2 and hmF2. Let us take the in situ Ne, hmF2, and NmF2 at the ME in the Northern Hemisphere as an example and compare the western peak of high electron densities (the schematic line of the electron density profile with the yellow line in Figure 7, line B1B2) with the electron density trough between the peaks (the schematic line of the electron density profile with the green line in Figure 7, line A1A2). Around longitudes 130° (western peak) at the ME in the Northern Hemisphere, hmF2 is relatively larger in Figure 6, so it means that the F2 peak in this region is much closer to 670 km (hmF2B > hmF2A); NmF2 is also relatively larger in Figure 4 (NmF2B > NmF2A). Both these two effects lead to a larger electron density at 670 km (NeB1 > NeA1). On the other hand, around the JS in the Southern Hemisphere, when hmF2 does not change much with longitude in Figure 6, the effect of NmF2 still works, and the in situ Ne in Figure 2 has similar patterns with NmF2 in Figure 4. The α-Chapman profile was used in some studies to fit the electron density profile with an altitude range of 170–600 km [e.g., Lei et al., 2005; Liu et al., 2007, 2011]. The α-Chapman profile with variable scale heights was also proposed as a possible choice for the topside profile model [Reinisch et al., 2007; Nsumei et al., 2012]. Verhulst and Stankov [2014, 2015] found that the best fit for the empirical modeling of the electron density profile in the topside ionosphere is the exponential profile with data recorded by the topside sounders on board the Alouette and ISIS satellites, followed by the Chapman profile. In the topside ionosphere where it

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Figure 5. Same as Figure 1 but for hmF2 from COSMIC.

is one or two scale heights higher than the F2 layer peak at middle latitudes, the diffusion equation (or the exponential profile) is probably more suitable than the Chapman profile. According to the diffusion equation, the topside Net at a height of h could be estimated by [Schunk and Nagy, 2009]   h  hmF2 (2) Net ¼ NmF2exp  Hp where Net is the topside electron density and Hp is the scale height. At a fixed height h in the topside ionosphere, such as A1 and B1 in Figure 7, Ne can be estimated from NmF2 and hmF2 at A2 and B2 with formula (2). In the diffusion approximation, the ions and electrons move together resulting in charge neutrality in a particular region. In a diffusive equilibrium regime, the major ion (or electron) density decreases exponentially with altitude at a rate governed by the plasma scale height as shown in formula (2) [Schunk and Nagy, 2009]. We calculated the plasma scale height (Ne decreases by a factor of 1/e) around 670 km with COSMIC electron density profiles and give the global distribution of geographical longitudinal variations of plasma scale height around 670 km in Figure 8. The longitudinal patterns of plasma scale height are dependent on electron temperature and ambipolar diffusion. Thus, the longitudinal variations of plasma scale height are different from those that depend on NmF2 and hmF2 behavior. The absence of wave-like longitudinal structures of scale heights at middle latitudes was also reported by Liu et al. [2008]. Because of this lack of correlation, we calculated the topside Ne at ~670 km from formula (2) with NmF2 and hmF2 from COSMIC with a constant scale height (the median values at different longitudes in the same latitude bands). The longitudinal patterns of the calculated Net are similar to the measured topside Ne at middle latitudes with the exception of DS in the Northern Hemisphere. Where there is a longitudinal pattern in NmF2, hmF2, or both, there is a longitudinal pattern in the topside Ne which is the convolution of the patterns in NmF2 and hmF2. Further work is necessary SU ET AL.

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Figure 6. Same as Figure 2 but for hmF2 from COSMIC. Notice that the scales of the Y axis in Figure 6 (left column) are different from those in Figure 6 (right column).

to find out the reason for a lack of similar longitudinal variations with other seasons in NmF2, hmF2, and topside Ne in the northern winter.

5. Discussion Why do the geographic longitudinal variations of hmF2 and NmF2 have such patterns? Several factors are frequently mentioned in previous studies: the geomagnetic field configuration, solar radiation, composition and temperature of the neutral atmosphere, winds, tides from below, and so on.

Figure 7. Schematic diagram for a possible cause of in situ Ne variations. The green and yellow lines are electron density profiles with different NmF2 and hmF2. Larger NmF2 and hmF2 leads to larger in situ Ne.

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The geomagnetic field is proposed to be responsible for the complicated longitudinal structures in the ionosphere [Zou et al., 2000; Rishbeth et al., 2000]. Yue et al. [2013] showed that magnetic field strength, declination, and dip angle have a zonal wave number 1-type structure in the Southern Hemisphere and zonal wave number 2-type structure in the Northern Hemisphere. The geomagnetic dip angle has a large curvature in southern middle latitudes. For instance, around a geomagnetic latitude of 60°S in the JS, the eastern geographic longitudes are sunlit at

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Figure 8. Same as Figure 1 but for the plasma scale height around 670 km from COSMIC.

10:30 LT while the western longitudes are in darkness, which creates a zonal wave number 1 type of electron density pattern at fixed geomagnetic latitudes. The O/N2 ratio also has larger values in eastern longitudes and a similar longitudinal variation [Qian et al., 2016]. The combined effect of the ionization by solar radiation, magnetic field configuration, and varying neutral composition is most likely responsible for the zonal wave number 1 type of pattern of NmF2 in the F2 region at southern middle latitudes. The winds may also influence the geographic longitudinal variations of NmF2. According to the study by Titheridge [1995], the mean lifetime of the ions exceeds 1 h at heights higher than 300 km. At these altitudes, Ne distribution becomes very dependent on dynamic effects caused by neutral winds and/or vertical drifts by electric fields. The combined effect of geomagnetic configuration and neutral winds could be used to explain the wave number 1 longitudinal pattern of Ne at southern middle latitudes and the wave number 2 pattern of Ne at northern middle latitudes measured by CHAMP at 400 km [Liu et al., 2010], the east-west coast differences of GPS TEC over the continental U.S. [Zhang et al., 2011], the east-west differences of Ne from the incoherent scatter radar measurements at Millstone Hill [Zhang et al., 2012], and the longitudinal variations of the nighttime Ne in the Southern Hemisphere [Luan et al., 2008; Luan and Dou, 2013]. The zonal winds are found to contribute to about 80% of the observed longitudinal dependence of Ne, whereas the meridional winds reduce the wind contribution to the longitudinal dependence to 65% over North America and Southern Ocean areas around the SE in a theoretical study [Wang et al., 2015]. Due to lack of wind measurements, the wind effects at middle latitudes are still uncertain. The causes for the geographic longitudinal variations of hmF2 and NmF2 still need more research. In addition to the effect of neutral winds, which has been the focus of many previous studies, the geographic longitudinal variations of topside Ne can be explained through many other processes, such as the geographic longitudinal variations of hmF2 and NmF2, although the causes of these variations are still not well understood. SU ET AL.

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In fact, the longitudinal patterns in hmF2 and NmF2 can be readily used to explain the longitudinal variations of the in situ topside Ne. NmF2 variations should directly connect to the changes in the topside Ne on the same field lines. Increases in the topside Ne should correspond to increases in NmF2, similarly decreases in topside Ne should relate to decreases in NmF2. The relationship between hmF2 and topside Ne is a little subtler and relates to the fact that some satellites flew at approximately a constant altitude to make these in situ measurements. The longitudinal changes of the F2 peak height can be as large as 15% for a total range of 30%. Even if we assume a very conservative value of hmF2 of 250 km, this represents a total change of hmF2 of 75 km. This will lead to very large changes in the in situ Ne measured by satellites flying through the topside. According to the diffusion equation of the topside ionosphere, a 10% increase of hmF2 of 250 km could lead to a 50% increase of Ne around 670 km. The increase or decrease of in situ measured Ne may be caused by the height of the in situ measurements that is closer to or farther away from the F2 layer peak. The in situ measurements of Ne in the topside can be affected by hmF2 in the following way. The topside electron density decreases exponentially above the F2 peak through the region of observations. If hmF2 is low, the observations are far above the F2 peak, so the exponential decrease is large and the in situ observed electron density will be small. If hmF2 is high, then there must be a much smaller exponential decrease of electron density, and the observed in situ Ne will be large. While this explanation may explain many of the in situ changes in electron density in the topside, it does not address the issue of why hmF2 changes. This will be the subject of future work. Thus, the main factor, which influences the geographic longitudinal variations of the topside Ne at fixed altitudes in the daytime, may be hmF2, which may have similar variation patterns as the in situ Ne in the data presented in this paper. In this case the secondary factor may be NmF2. When hmF2 does not change much, the geographic longitudinal variations of in situ Ne may be similar to NmF2. There are also some other factors that are still unknown. One reason for thinking this is that we do not have an explanation for the situation around DS in the Northern Hemisphere. The topside ionosphere contains several ion species; the electron densities are the sum of the densities of all of these ion species. However, the effects of the major ions and the minor ion species should be considered separately. We looked at the densities of O+, He+, and H+ from DEMETER IAP (Instrument Analyseur de Plasma) measurements at 670 km in 2007–2009 in the daytime. The O+ ions are the main ions at this altitude, which is also reported by Zhang et al. [2015]. The light ions make up a minuscule contribution to the total ion density at these altitudes. These results are consistent with that the transition height reached 850 km during the daytime in 2008 [Heelis et al., 2009]. The longitudinal variation of the O+ density is similar to that of the electron density. The longitudinal variations of the light ions are totally different from those of O+ ions and electrons when the same processing methods are used (see the earlier description of these methods). The influences of light ions on the longitudinal variations of the electrons at 670 km in 2007–2009 in the daytime appear to be small.

6. Summary The causes of longitudinal variations of electron density in the topside ionosphere are still poorly understood, although they have been studied for decades. From the comparisons of the geographic longitudinal variations of in situ Ne measured at a fixed height by a circular orbiting satellite, and hmF2 and NmF2 from COSMIC, we found the following: 1. In the topside ionosphere, the longitudinal variations of Ne in the Southern Hemisphere are much larger than those in the Northern Hemisphere. There are gradual changes of geographic longitudinal variations with geomagnetic latitude in all the seasons. The longitudinal patterns around the DS in the Northern Hemisphere (northern winter) are different from other seasons in 2007–2009, whereas the patterns in each season in the Southern Hemisphere are similar, although the longitudinal locations of these peaks have seasonal variations. In the Northern Hemisphere, the zero crossing of longitudinal variations coincides with the sign changes of magnetic declination in most seasons except in northern winter (DS). In the Southern Hemisphere, the pattern of one density enhancement zone and one density reduction zone is consistent with the longitudinal variation of magnetic declination, but the locations of zero crossing are different from the zero crossing of magnetic declination for all seasons.

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2. Around the ME, JS, and SE in the Northern Hemisphere and around the ME, SE, and DS in the Southern Hemisphere, the patterns of the longitudinal variations of in situ topside Ne and hmF2 are consistent with each other. 3. Around the JS in the Southern Hemisphere (southern winter), the in situ topside Ne has similar longitudinal patterns to NmF2 and hmF2 does not change much with longitude. The midlatitude ionosphere, including the topside, is under strong influences from the neutral winds, electric fields, diffusion, and other basic dynamical processes, as well as from the neutral composition and temperature. These influences result in a variety of changes in hmF2, NmF2, and the ionospheric topside. These variations are correlated to each other. The topside variations may be explained intuitively in terms of hmF2 and NmF2. This approach works reasonably well in most of the situations that arose in the current paper. This applies to observations in the topside not too far from the F2 peak but not to observations that are higher up. In this sense, understanding variations in hmF2 and NmF2 becomes an important and relevant subject for topside ionospheric studies. However, under certain conditions, this hmF2 and NmF2 analysis approach appears to have oversimplified the complicated influences from the fundamental physical and chemical procedures. The cause of the winter variations of topside Ne with longitude and the cause of the longitudinal variations of hmF2 and NmF2 will be explored in future work. Acknowledgments We are grateful to the Centre National d’Etudes Spatiales (CNES, http://missions-scientifiques.cnes.fr/DEMETER/ index.htm) for the DEMETER data, the Taiwan’s National Space Organization and the University Corporation for Atmospheric Research for the COSMIC data (http://www.cosmic.ucar.edu/), the World Data Center for the Kp index (http://wdc.kugi.kyoto-u.ac.jp/kp/index. html), and the National Oceanic and Atmospheric Administration for the F10.7 index (ftp.ngdc.noaa.gov). This research was supported by the National Natural Science Foundation of China (41204107 and 41304047). The National Center for Atmospheric Research is sponsored by the NSF.

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