Simulated midlatitude summer nighttime anomaly in realistic ...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, A03323, doi:10.1029/2011JA017010, 2012

Simulated midlatitude summer nighttime anomaly in realistic geomagnetic fields Zhipeng Ren,1 Weixing Wan,1 Libo Liu,1 Huijun Le,1 and Maosheng He1 Received 15 July 2011; revised 20 January 2012; accepted 20 January 2012; published 17 March 2012.

[1] In the present work, using the three-dimensional theoretical ionospheric model of the Earth in Institute of Geology and Geophysics, Chinese Academy of Sciences (TIME3D-IGGCAS), we simulate the temporal-spatial distribution of the midlatitude summer nighttime anomaly (MSNA) and the influences of thermospheric meridional and zonal winds on the formation of MSNA. MSNA mainly appears in three distinct regions, the East Asian region, the northern Atlantic-Europe region and the South Pacific region. MSNA in the South Pacific region is obviously stronger than that in the other two regions. In these three regions, MSNA not only occurs in local summer, but also often occurs in Equinox. Through lifting up or lowering the F region ionization, thermospheric winds mainly drive the formation of MSNA. In the East Asian region and the northern Atlantic-Europe region, MSNA are mainly driven by the thermospheric meridional wind, and the influence of the thermospheric zonal wind can be neglected. However, although MSNA in the South Pacific region (or Weddell Sea Anomaly) are mainly driven by the thermospheric meridional wind, the thermospheric zonal wind also plays an important role in the formation of MSNA. Citation: Ren, Z., W. Wan, L. Liu, H. Le, and M. He (2012), Simulated midlatitude summer nighttime anomaly in realistic geomagnetic fields, J. Geophys. Res., 117, A03323, doi:10.1029/2011JA017010.

1. Introduction [2] During the international geophysical year in 1957, Bellchambers and Piggott [1958] first found that the diurnal maximum of foF2 at Faraday (65.2°S, 64.6°W) occurs in the evening or at night in local summer. Similar phenomenon was also found to occur in the other regions around the Weddell Sea [e.g., Rastogi, 1960; Penndorf, 1965; Dudeney and Piggott, 1978]. Historically, any F2 layer behaviors departure from the solar zenith angle dependence as predicted by the Chapman ionization theory has been called an “anomalies.” Thus, these unusual nighttime enhancements are named as the “Weddell Sea Anomaly” (WSA). Recent researches suggested that similar anomaly can also be found in the other longitudinal sectors of Southern Hemisphere and in Northern Hemisphere [e.g., Lin et al., 2009; Thampi et al., 2009, 2011]. Thus, recently, this kind of anomalous electron density patterns in both Northern and Southern Hemispheres are called midlatitude summer nighttime anomaly (MSNA) [e.g., Thampi et al., 2009, 2011]. Using the modern satellite observations, detailed climatology of the MSNA (or WSA) has been further investigated [e.g., Horvath and Essex, 2003; Horvath, 2006; Burns et al., 2008; He et al., 2009; Jee et al., 2009; Lin et al., 2009; Liu et al., 2010, and references therein]. Horvath and Essex [2003], Horvath [2006] and Jee 1 Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China.

Copyright 2012 by the American Geophysical Union. 0148-0227/12/2011JA017010

et al. [2009] investigated the two-dimensional distributions of the MSNA using TEC measurements over the oceans collected by the TOPEX/Poseidon. Burns et al. [2008], Lin et al. [2009] and He et al. [2009] further studied the evolution of the three-dimensional structure of the MSNA based on the vertical ionospheric density profiles observed by the COSMIC. Liu et al. [2010] studied the details of the MSNA and its physical mechanisms using in situ measurements from the CHAMP satellite. Not only the climatology of MSNA but also its physical mechanism has been studied in detail in recently work. [3] The physical mechanisms for MSNA (or WSA) formation have been studied for several decades. Earlier, the WSA was suspected to be connected with high-latitude convection patterns [e.g., Penndorf, 1965]. However, the Weddell Sea region is in at the magnetic middle latitude. Previous researchers noticed that WSA take place in the longitude sectors where the dip equator excurses farthest toward the geographic pole, and suggested that the combined effect of the longer summer sunlight hours and the day-to-night/summer-to-winter equatorward neutral winds play important roles in the formation of WSA [Kohl and King, 1967; Rishbeth and Garriott, 1969; Dudeney and Piggott, 1978; Horvath and Essex, 2003]. With a series of simulations, Rishbeth [1967, 1968] showed that with the effect of the equatorward wind, NmF2 tends to increase at midlatitudes at summer dusk. Dudeney and Piggott [1978] also proposed the thermospheric wind mechanism which ascribes the WSA to the interaction of solar photoionization, thermospheric winds and the geomagnetic field. Recent

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researches also supported that the WSA is mainly driven by the thermospheric winds [e.g., Horvath and Essex, 2003; Horvath, 2006; He et al., 2009; Lin et al., 2009; Liu et al., 2010]. Liu et al. [2010] studied the MSNA based on the CHAMP observations, and suggested that the neutral wind combined with the geomagnetic field configuration play a pivotal role in the formation of the MSNA. Thampi et al. [2011] and Chen et al. [2011] simulated the MSNA (or WSA), and also suggested that the major physical mechanisms for the MSNA formation are the equatorward neutral wind. However, using COSMIC data, Burns et al. [2008] studied the ionosphere over the South Pacific at summer dusk and discussed characteristics of various mechanisms about the WSA in detail. They pointed out that if the zonal wind prevails around dusk as suggested by Dickinson et al. [1981], the wind process would not be significant at driving the WSA. In addition to this neutral wind process another mechanism can also affect the WSA. For example, Evans [1965] suggested that, the rapid fall of electron temperature at sunset cause the downward motion of the ionization from above the F2 peak and from the plasmasphere, and leads to an increasing of plasma near and above the F2 peak. There are still several other potential mechanisms, including the transportation of plasma from the dayside ionosphere, the downward flux from the plasmasphere, and so on. However, Liu et al. [2010] pointed out that the MSNA are obvious in three distinct regions, the East Asian region, the northern Atlantic region and the South Pacific region, where the geomagnetic field declinations are obviously larger than that in the other region. Thus, the geomagnetic fields should play an important role in the formation of the MSNA. [4] It is now generally accepted that the geomagnetic fields have obviously influences on the ionosphere, and mainly have two consequences modifying the ionosphere. First, as the geomagnetic field line decides the direction of the field-aligned diffusion, diffusions driven by the neutral winds, by plasma pressure gradient, and by gravity all depend on the geomagnetic fields, and the geomagnetic field obviously modify the velocity of the ionospheric plasma field-aligned diffusion. Second, through modifying the ionospheric dynamo, the geomagnetic fields can affect the ionospheric electrodynamic drifts [e.g., Ren et al., 2009a], which perpendiculars to the geomagnetic fields. Through these two consequences, the geomagnetic fields obviously affect the ionospheric plasma density, plasma composition, electron and ion temperature [e.g., Rishbeth, 1998; Ren et al., 2008a, 2009a]. The simulations play important roles in the research of MSNA. Previous simulations mainly study the physical mechanism or regional character. For example, Chen et al. [2011] mainly studied the WSA (South Pacific region), and Thampi et al. [2011] simulated the MSNA in East Asian region. However, the geomagnetic fields have been simply treated as dipole fields in many theoretical ionospheric models and previous simulations, and most of modeling researches on MSNA or WSA had used dipole fields. Dipole field is a good approximation for the geomagnetic fields at low latitudes and midlatitudes. By fitting a magnetic dipole field to the IGRF field in the appropriate longitude plane, Chen et al. [2011] had successfully simulated WSA based on proper magnetic declination. However, dipole field could not well describe the large North-South

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asymmetry in geomagnetic fields. Hence, it is very difficult to simulate Northern Hemisphere and Southern Hemisphere well at the same time in a dipole field. Here, we will try to simultaneously simulate the global MSNA in realistic geomagnetic fields based on Three-Dimensional Theoretical Ionospheric Model of the Earth in Institute of Geology and Geophysics, Chinese Academy of Sciences (TIME3DIGGCAS), and analyze the temporal-spatial distribution and physical mechanism of MSNA.

2. TIME3D-IGGCAS Model and Inputs [5] In many theoretical ionospheric models, the geomagnetic fields have been simply treated as dipole fields. Although dipole field is a good approximation for the geomagnetic fields at low latitudes and midlatitudes, the nondipole composition obviously affects the distributions of ionospheric parameters. Some ionospheric models had successfully simulated ionospheric processes in realistic geomagnetic fields frame. For example, based on a spherical geographical coordinate system, TIEGCM (thermosphereionosphere general circulation model) and GCITEM-IGGCAS (Global Coupled Ionosphere-Thermosphere-Electrodynamics Model developed at Institute of Geology and Geophysics, Chinese Academy of Sciences) can simulate the ionosphere in the realistic geomagnetic field [Roble et al., 1988; Ren et al., 2009b]. However, this kind of models could not selfconsistently calculate the heat flux and plasma flux at the upper boundary, and had to obtain these flux from the other model. Theoretical ionospheric model based on the closed geomagnetic tubes can self-consistently calculate these fluxes. However, it is very difficult to add the realistic geomagnetic field into this kind of model. To describe the influence of the realistic geomagnetic field, Richmond [1995] developed the ionospheric dynamo equations for a general geomagnetic field configuration, with specific application to coordinate systems based on magnetic APEX coordinates [VanZandt et al., 1972]. Using similar method, a series of theoretical models based on the closed geomagnetic tubes achieve the simulation in the nondipole geomagnetic field [e.g., Millward et al., 2007; Ren et al., 2008b]. On the basis of a similar method, a new three-dimension (3-D) midlatitude and low-latitude theoretical ionospheric model (TIME3D-IGGCAS) in realistic geomagnetic fields had been developed [Ren et al., 2012]. [6] Yue et al. [2008] had developed a two-dimension midlatitude and low-latitude theoretical ionospheric model (TIME-IGGCAS). This model is steady and credible, and can reproduce most large-scale features of ionosphere. However, TIME-IGGCAS uses an eccentric dipole geomagnetic field, and could not describe the realistic geomagnetic field very well. Through using magnetic APEX coordinates, TIME3D-IGGCAS can successfully simulate 3-D ionospheric processes in realistic geomagnetic fields frame. TIME3D-IGGCAS covers the midlatitude and lowlatitude ionosphere and whole plasmasphere and gives information from 130 km to higher than 20,000 km. The simulations will be carried out on 24 magnetic meridional planes, and 35 magnetic field lines are included in every plane. The horizontal resolution in the low-latitude region is about 1° in magnetic latitude and 15° in magnetic longitude. TIME3D-IGGCAS also solves the coupled

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equations of the mass continuity, momentum, and energy in closed geomagnetic tubes, and can provide the densities, temperatures and field-aligned diffusion velocities of three main ions O+, H+, He+ and electrons. Under the assumption of photochemical equilibrium, the densities of N+2 , O+2 and NO+ are also calculated. TIME3D-IGGCAS ignores the differences between the temperatures of the different ions, and only the average ion temperature is calculated. The numerical procedure and the choosing of parameters such as heating rates and collision frequency of TIME3DIGGCAS are the same as that used in TIME-IGGCAS [Yue et al., 2008]. The principal input parameters for TIME3DIGGCAS model, such as the neutral winds, neutral temperature neutral number densities, and EB plasma drifts and are mainly obtained from observations or from empirical models.

3. Result and Discussions 3.1. Distribution of the MSNA Region [7] To study the distribution of the MSNA region, we simulate ionospheric diurnal variations at June solstice and at December solstice. These two simulations both are performed for low solar flux level, corresponding to a solar 10.7 cm flux index (F10.7) of 70, and geomagnetic quiet inputs with an Ap index of 4. The empirical models provide the neutral temperature, neutral compositions and neutral winds in these two simulations. The neutral temperature and neutral number density of O2, N2, O, He and H are from NRLMSIS-00 empirical model, and the empirical model of Titheridge [1997] provides the neutral number density of NO. HWM-93 empirical model provides the neutral winds. The meridional and zonal electrodynamics drifts in these simulations are determined by the ionospheric electric fields from Richmond empirical model [Richmond et al., 1980]. The initial conditions of the electron temperature, ion temperature, ion number densities of O+, O+2 , NO+, H+, He+ and electron are from IRI empirical model. These two simulations are carried out on 24 magnetic meridional planes, and 35 magnetic field lines are included in every plane. The time steps in these simulations are 300 s, and three model day runs were made to obtain the presented results. [8] Figure 1 shows the simulated F region peak electron number density (NmF2), which is in units of m3. The first panel shows the geographical latitudinal and longitudinal variations of the NmF2 at noon (12:00 LT), and the second panel shows the geographical distribution of NmF2 at night (20:00 LT). The left column shows NmF2 at June solstice, and the right column shows NmF2 at December solstice. To compare with each other, these four plots use the same color scale. At both 12:00 LT and 20:00 LT, there is obvious equatorial ionization anomaly (EIA) at equatorial and lowlatitude regions in two solstices. However, the night EIA in winter hemisphere is very weak, and nearly disappears in some longitudinal sectors. Comparing NmF2 in two solstices, it can be concluded that the values are bigger in December solstice than that in June solstice. This phenomenon is called ionospheric annual anomaly. For middle latitude, the NmF2 of winter hemisphere is bigger than that of summer hemisphere during daytime in two solstices, whereas it is opposite in nighttime. This feature accords with the “ionospheric

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winter anomaly.” We also notice that daytime NmF2 in southern America and southern Atlantic region is higher than that in the other longitudinal sectors at Southern Hemisphere in both solstices, and that night NmF2 in South Pacific region is higher than that in the other longitudinal sectors at Southern Hemisphere in both solstices. [9] To analyze the details of the MSNA, we also show the differences between NmF2 at 20:00 LT and that at 12:00 LT (D NmF2 = NmF2 20:00LT NmF2 12:00LT) at the third panel of Figure 1. The left plot shows DNmF2 at June solstice, and the right plot shows DNmF2 at December solstice. Note that different color scales from above plots are used to clearly show the details of these two patterns. The APEX latitude of the black lines in these plots is 45° at the height of 320 km when the reference altitude is 89 km. As shown in these plots, DNmF2 in winter hemisphere are mainly negative, indicating higher NmF2 at day than at night. However, there are two regions in the summer hemisphere in both solstices with positive DNmF2, signaling a diurnal anomaly with higher NmF2 at night than at day. The first region is equatorial anomaly crest in the summer hemisphere, and the second region is the summer middle latitude regions. We should notice that, although DNmF2 in these two regions show similar amplitude, the relative variations of NmF2 in middle latitude are obviously large than that in equatorial anomaly crest. Recently, this kind of anomaly in the summer middle latitude regions is called midlatitude summer nighttime anomaly (MSNA) [e.g., Lin et al., 2009; Thampi et al., 2009, 2011]. We notice that MSNA mainly appear in three regions. Two such anomaly regions exist in Northern Hemisphere in June solstice. One region is between 90°E longitude and 150°E longitude in East Asia region, and the other region is between 280°E longitude and 50°E longitude in North Atlantic – Europe region. In the centers of these two regions, NmF2 at 20:00 LT can be about 50%100% larger than that at 12:00 LT. In December solstice, one such anomaly region also exists between 180°E longitude and 300°E longitude in South Pacific region. MSNA in this region is obviously stronger than that in the other two regions, and NmF2 at 20:00 LT can be nearly 150% larger than that at 12:00 LT in the center of South Pacific region. This region clearly tilts following the excursion of the dip equator, which may imply its connection to the geomagnetic field. Liu et al. [2010] found similar distribution of MSNA based on the CHAMP observations. They pointed out that the MSNA are obvious in three distinct regions, the East Asian region, the northern Atlantic region and the South Pacific region, where the geomagnetic field declinations are obviously larger than that in the other region, and suggested that the neutral wind combined with the geomagnetic field configuration play a pivotal role in the formation of the MSNA. To examine above suggestions, in the bottom panel, we also show the geomagnetic declinations along the black lines (third panel). The left plot shows the geomagnetic declinations at Northern Hemisphere, and right plot shows the geomagnetic declinations at Southern Hemisphere. As shown in these plots, the geomagnetic declinations show peaks in above three distinct regions, the declinations in Northern Hemisphere MSNA region are mainly negative, and the declinations in Southern Hemisphere MSNA region

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Figure 1. Geographical distribution of NmF2 (in units of m3) at (first panel) noon (12:00 LT) and at (second panel) night (20:00 LT), (third panel) the difference between them, (fourth panel) the geomagnetic declinations along the black lines in the plots of the third panel, and for (left) June solstice and for (right) December solstice. Note that different color scales are used to clearly show the relevant patterns in different plot. are mainly positive. Hence, the geomagnetic field configuration plays an important role in the formation of the MSNA. 3.2. Intra-annual Variations of MSNA [10] After analyzing the spatial distribution of the MSNA, we should also simulate the temporal variability of the MSNA. With a series of simulations, we get the diurnal dependences of middle- and low-latitude ionosphere for different months. To show the details of the temporal variability of MSNA in different regions, we choose three locations near the center of above three anomaly regions. The geographical coordinates of these three points are respective 45°N, 120°E (East Asian region), 40°N, 330°E (northern Atlantic region), and 60°S, 270°E (South Pacific region). Figure 2 shows intra-annual and diurnal variations of NmF2 and F region peak height (hmF2) of these three points. The left column shows the NmF2 in units of m3. To compare with each other these three plots use the same color scale. The right column shows the hmF2 in units of kilometer (km). To compare with each other, these three plots also use the same color scale. The top plots show the simulated results in the East Asian region, the middle plots show the simulated results in the northern Atlantic region, and the

bottom plots show the simulated results in the South Pacific region. As shown in these plots, MSNA occurs not only in local summer, but also in the other seasons. [11] In East Asia region (45°N, 120°E) and northern Atlantic region (40°N, 330°E), NmF2 maximizes near 14:00 LT and decreases toward both dawn and dusk sides near December solstice (local winter). This is the normal ionospheric diurnal variations predicted by the Chapman ionization theory. However, in June solstice (local summer), NmF2 maximizes near 19:00 LT, and the maximum NmF2 is about 50% larger than that at noon. We should notice that higher nighttime NmF2 lasts for only about 3 h after sunset. Similar phenomena are also found by Liu et al. [2010] in East Asia region based on the electron density observed by CHAMP at 400 km. As shown in these plots, similar diurnal variations can also be found throughout April–August. Furthermore, the NmF2 maximum seems to shift toward later local times from January toward May, back to earlier local times from September toward December. In northern Atlantic region, there are a second NmF2 maximum near sunrise between February and April. However, the diurnal variations of hmF2 in different months are very similar with each other. They also show an obviously diurnal variation,

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Figure 2. Intra-annual and diurnal variations of (left) NmF2 (in units of m3) and (right) hmF2 (in units of km) in (top) the East Asian region (45°N, 120°E), in (middle) the northern Atlantic region (40°N, 330°E), and in (bottom) the South Pacific region (60°S, 270°E). Note that the same parameters use the same color scale. which is lower during daytime and higher at night. The night hmF2 mainly show a semiannual variation, and the daytime hmF2 mainly show an annual variation. [12] In South Pacific region (60°S, 270°E), the anomalous NmF2 diurnal variations mainly occurs during September– March, with NmF2 maximizes at night. We notice that higher nighttime NmF2 can last to midnight or even throughout the night. In comparison, high values of NmF2 last only for only about 3 h in above two regions. A second NmF2 maximum presents at morning in these months. From October to February (local winter), NmF2 minimizes around noon. In local summer (May–August), NmF2 maximizes near noon (12:00 LT) and decreases toward both dawn and dusk sides. Similar to above two regions, the maximum NmF2 has a tendency to shift toward earlier local times from January toward June, back to later local times from June toward December. Although the diurnal variations of NmF2 in South Pacific region show some differences from the other two regions, the diurnal variations of hmF2 do not show obvious differences. In different months hmF2 also shows an obviously diurnal variation, which is lower during daytime and higher at night.

3.3. Influence of Thermospheric Winds on MSNA [13] After analyzing the temporal-spatial distribution of MSNA, we pay attention to the source of MSNA. Recent researches suggested that the MSNA is mainly driven by the thermospheric winds [e.g., Horvath and Essex, 2003; Horvath, 2006; He et al., 2009; Lin et al., 2009; Liu et al., 2010; Chen et al., 2011]. Liu et al. [2010] found that the MSNA are obvious in three distinct regions, where the geomagnetic field declinations are obviously larger than that in the other region, and suggested that the neutral wind combined with the geomagnetic field configuration play a pivotal role in the formation of the MSNA. Thus, we will mainly simulate and discuss the influence of thermospheric winds on the formation of MSNA in this subsection. [14] We simulate ionospheric diurnal variations at June solstice and at December solstice in four conditions: (1) with all thermospheric winds, (2) only with thermospheric meridional wind, (3) only with thermospheric zonal wind, and (4) without any thermospheric winds. We show the diurnal variations of NmF2 and hmF2 at above three points for local summer in Figure 3. The left column shows the diurnal

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Figure 3. Diurnal variations of (left) NmF2 (in units of m3) and (right) hmF2 (in units of km) in (top) the East Asian region (45°N, 120°E) at June solstice, in (middle) the northern Atlantic region (40°N, 330°E) at June solstice, and in (bottom) the South Pacific region (60°S, 270°E) at December solstice. Black lines show the diurnal variations with the influences of all thermospheric winds, green lines show that only with the influences of thermospheric meridional wind, blue lines show that only with the influences of thermospheric zonal wind, and red lines show that without any thermospheric winds’ influences. variations of NmF2 in units of m3, and the right column shows the diurnal variations of hmF2 in units of km. Black lines in these plots show the diurnal variations with the influences of all thermospheric winds, green lines show that only with the influences of thermospheric meridional wind, blue lines show that only with the influences of thermospheric zonal wind, and red lines show that without any thermospheric winds’ influences. The top plots show the diurnal variations in the East Asian region (45°N, 120°E) at June solstice, and the middle plots show that in the northern Atlantic region (40°N, 330°E) at June solstice. As shown in these plots, in both locations, NmF2 in case 1 and 2 show similar diurnal variations, and NmF2 in case 3 and 4 show similar diurnal variations. Although weak MSNA phenomena also occur in case 3 and 4, we notice that MSNA phenomenon in case 1 and 2 are obviously stronger than that in case 3 and 4. In the East Asian region, NmF2 at 20:00 LT is about 4% larger than that at 12:00 LT in case 4 (red lines), but NmF2 at 20:00 LT is about 50% larger than that at 12:00 LT in case 1 (black lines). In the northern Atlantic region, NmF2 at 20:00 LT is about 15% larger than that at 20:00 LT in case 4, but NmF2 at 20:00 LT is about 100% larger than that at 12:00 LT in case 1. Thus, although the other mechanisms also affect its generation, this simulated results suggested that MSNA in these two regions are mainly driven by the thermospheric meridional wind, and the influence of the thermospheric zonal wind can be neglect. As shown in the diurnal variations of hmF2, thermospheric meridional

wind lifts up the height of F region in both regions in most of local time, and NmF2 also increase with the lifting of the F region height. Thus, by lifting up the F region ionization, the thermospheric meridional wind drives MSNA phenomena in these two regions. [15] The bottom plots show the diurnal variations in the South Pacific region (60°S, 270°E) at December solstice. As shown in these plots, thermospheric meridional wind and zonal wind both drive the MSNA phenomenon in this region, and the influence of meridional wind is obviously stronger than that of zonal wind. The thermospheric meridional wind mainly increases NmF2, and its influence in South Pacific region is also stronger than that in the other two regions. In the other two regions, the NmF2 maximums in case 2 are about 25%50% higher than that in case 4. However, in the South Pacific region, the NmF2 maximum in case 2 is twice of that in case 4. Although the influence of zonal wind is obviously weaker than that of meridional wind, the NmF2 maximum in case 3 is also 50% higher than that in case 4, and the influence of zonal wind in the South Pacific region can compare with that of meridional wind in the other two regions. The zonal wind mainly decreases NmF2 between 06:00 LT and 16:00 LT, and increase NmF2 at the other periods. [16] However, although NmF2 in the South Pacific region in case 3 and 4 only show one maximum, there a second maximum of NmF2 near 06:00 LT in case 1 and 2. Actually, we notice that hmF2 at night in case 1 and 2 also show two

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is obviously stronger than that in the other two regions. In these three regions, MSNA not only occurs in local summer, but also often occurs in Equinox. MSNA are mainly driven by thermospheric winds. In the East Asian region and the northern Atlantic–Europe region, MSNA are mainly driven by the thermospheric meridional wind, and the influence of the thermospheric zonal wind can be neglect. However, although MSNA in the South Pacific region (or WSA) are mainly driven by the thermospheric meridional wind, the thermospheric zonal wind also plays an important role in the formation of MSNA. Through lifting up or lowering the F region ionization, thermospheric wind affect the formation of MSNA.

Figure 4. Diurnal variations of Weff (in units of m/s) in the South Pacific region (60°S, 270°E) at December solstice. Black lines show the diurnal variations with the influences of all thermospheric winds, green lines show that only with the influences of thermospheric meridional wind, and blue lines show that only with the influences of thermospheric zonal wind. maximums. As NmF2 and hmF2 at night in case 3 and 4 only show one peak, the second maximum may mainly drive by the meridional wind., the thermospheric meridional and zonal winds may drive these peaks by lifting up the F region ionization. To analyze its mechanism, we need to calculate the actual influence of the thermospheric wind (Weff). Liu et al. [2010] expressed this parameter as Weff ¼ ðV cos D þ U sin DÞ cos I sin I:

ð1Þ

Here, U and V are respective the zonal (eastward) and meridional (northward) winds, D and I be respective the magnetic declination and inclination angles. Weff obviously depends on the configuration of the geomagnetic fields, and larger Weff suggests that the thermospheric wind have a significant component parallel to the geomagnetic fields. Figure 4 shows the diurnal variations of Weff in the South Pacific region (60°S, 270°E) at December solstice. Black lines show the diurnal variations with the influences of all thermospheric winds, green lines show that only with the influences of thermospheric meridional wind, and blue lines show that only with the influences of thermospheric zonal wind. As shown in this figure, Weff show two upward peaks before two maximums of NmF2. The first peak is driven by meridional and zonal winds, and the second peak is driven by only meridional wind.

4. Summary and Conclusion [17] In the present work, using the TIME3D-IGGCAS model, we simulate the temporal-spatial distribution of the midlatitude summer nighttime anomaly (MSNA), and influences of thermospheric meridional and zonal winds on the formation of MSNA. MSNA mainly appears in three distinct regions, the East Asian region, the northern Atlantic–Europe region and the South Pacific region, where the geomagnetic field declinations are obviously larger than that in the other region. MSNA in the South Pacific region

[18] Acknowledgments. This work is supported by National Science Foundation of China (41004070, 40974090, 41131066, 41174137), National Important Basic Research Project (2011CB811405), Knowledge Innovation Program of the Chinese Academy of Sciences, and China Postdoctoral Science Foundation. The author gratefully acknowledges the support of K. C. Wong Education Foundation, Hong Kong. [19] Robert Lysak thanks the reviewers for their assistance in evaluating this paper.

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