3. Results and Discussion
Figure 2 and
Figure 3 show the global distributions of relative longitudinal variation (Δ
ρr) from APOD in different months at dawn and dusk, respectively. The most prominent feature is one region with high Δ
ρr in each hemisphere. In the northern hemisphere, the zonal maximum of Δ
ρr (Δ
ρrmax) appears at 60–100° W at dawn except for 40° W in June and appears at 80–120° W at dusk except 60° W in November. In the southern hemisphere, Δ
ρrmax appears at 80–120° E at dawn except 60° E in January and appears at 160–200° E at dusk except 140° E in February. From November to February, the high-density region in the northern hemisphere is more pronounced. The global Δ
ρrmax appears at 65–75° N in latitude and 60–120° W in longitude. From April to September, the high-density region in the Southern Hemisphere is more pronounced, and Δ
ρrmax is located at 50–75° S and 100–180° E. The location of Δ
ρrmax is close to the geomagnetic pole, which was at (~83° N, ~84° W) in the northern hemisphere and (~75° S, ~125° E) in the Southern Hemisphere in 2017–2018, according to the altitude-adjusted corrected geomagnetic (AACGM) coordinates [
24]. It is known that auroral heating occurs mainly around the geomagnetic pole [
25,
26,
27,
28], which causes the enhancement of temperature (thermospheric density) in the lower (upper) thermosphere [
29]. Thus, the maximum of Δ
ρr occurring near the geomagnetic pole can be attributed to the aurora heating, including the aurora particles precipitation and Joule heating.
Figure 2 and
Figure 3 show that the maximum of Δ
ρr in the Southern Hemisphere is greater than in the Northern Hemisphere at both dawn and dusk from April to August. During November and February, the maximum of Δ
ρr in the Northern Hemisphere is greater than in the Southern Hemisphere. For example, in December, the maximum of Δ
ρr in the Northern Hemisphere is 13.2% and 25.4% at dawn and dusk, respectively, while the maximum in the Southern Hemisphere is only 9.0% and 7.1% at dawn and dusk, respectively. It indicates that Δ
ρrmax in the winter hemisphere is higher than in the summer hemisphere around the solstices. The difference of Δ
ρrmax between the summer and winter hemispheres may be caused by the difference in the solar EUV energy input into the thermosphere between the two hemispheres. At the same latitude in two hemispheres, the solar elevating angle in the winter hemisphere is smaller than in the summer, and some polar regions in the winter hemisphere are not even lit by the Sun. Therefore, the EUV energy input into the thermosphere and
in the winter hemisphere are much less than those in the summer hemisphere, which causes the lower background thermosphere density in the winter hemisphere. According to Equation (2), Δ
ρr is inversely proportional to the value of
. Thus, Δ
ρrmax caused by the auroral heating in the winter hemisphere was more significant than in the summer hemisphere.
In
Figure 2 and
Figure 3, the Δ
ρrmax maximizes annually at 26.3% and 39.6% in July in the Southern Hemisphere near the geomagnetic pole (~75° S, ~125° E) at dawn and dusk, respectively. The annual maximum of Δ
ρrmax in the Northern Hemisphere appears in February and December at dawn and dusk, with values of 15.8% and 25.4%, respectively. The annual maximum in the Southern Hemisphere is much greater than in the Northern Hemisphere. The difference in Δ
ρrmax between the two hemispheres should be mainly caused by the different geomagnetic pole positions relative to the geographic poles. Since the aurora heating is mainly around the geomagnetic pole [
25,
26,
27,
28] and the southern geomagnetic pole is further off the geographical pole, the effects of auroral heating on the thermosphere in the Southern Hemisphere are harder to cover all longitudes. Thus, the longitudinal variation of thermospheric density in the Southern Hemisphere should be relatively stronger in the Northern Hemisphere under the same other conditions. Xu et al. [
10] analyzed the longitudinal variation of thermospheric density using the CHAMP and GRACE satellite observations. Their results showed that the maximal longitude variations averaged for all local times also appear near the geomagnetic poles. Similar to the APOD observations, the CHAMP and GRACE satellite observations showed an apparent hemispheric asymmetry in the longitudinal structure, more pronounced in the Southern Hemisphere than in the Northern Hemisphere. To sum up, the main feature of the global distribution around the terminator from APOD is similar to the distribution averaged over all local times from GRACE.
There is a low-density region in each hemisphere, which is close to the high-density region in latitude and far away from the high-density region in longitude. In the Northern Hemisphere, the minimum of Δρr (Δρrmin) appears at 80–160° E at dawn and appears at 80–140° W at dusk. In the Southern Hemisphere, Δρrmin appears at 60–100° W at dawn except for 40° W in November and appears at 0–40° E at dusk except for 60° E in December. Δρrmin in the Southern Hemisphere is less than in the Northern Hemisphere at both dawn and dusk from April to August. However, from November to February, the minima of Δρr in the Northern Hemisphere is less than in the Southern Hemisphere, which is in summer. For example, in the December Northern Hemisphere, the Δρr minimizes at −11.3% and −20.4% at dawn and dusk, respectively, while in the Southern Hemisphere it minimizes at −7.8% and −8.4% at dawn and dusk, respectively.
As is shown in
Figure 2 and
Figure 3, the longitudinal variations of Δ
ρr around the geomagnetic pole significantly expand to the middle and low latitudes. The expansion diminishes with latitude decreasing, and the values of Δ
ρr at low latitudes vary between −10% and 10% in most months. The expansion also changes with the seasons. Near the solstices, the longitudinal variation around the geomagnetic pole in the summer hemisphere can control the low latitudes and extend to the other hemisphere. Otherwise, the longitudinal variations around the geomagnetic pole in the winter hemisphere have weaker impacts on the mid-low latitudes, although the maxima of Δ
ρr in the winter hemisphere are larger. The difference in equatorward expansion could be related to the meridional wind in the mid-low latitudes. To clarify the contribution of meridional wind to the equatorward expansion and the asymmetry of Δ
ρrmax between the two hemispheres, we calculated the meridional wind in the middle and high latitudes at dawn (0730 LT) and dusk (1930 LT) using the empirical model HL-TWiM. The seasonal distribution of meridional wind between 30–80° N at 84° W and 30–80° S at 125° E is given in the upper panel of
Figure 4. According to
Figure 4, during the solstices, the thermospheric prevailing meridional wind is equatorward in the summer hemisphere and at latitudes 30–40° N (S) in the winter hemisphere. The equatorward wind should facilitate the longitude variations of Δ
ρrmax around the magnetic pole in the summer hemisphere extending to low latitudes. It may help reduce the value of Δ
ρrmax in the summer hemisphere.
Figure 2 and
Figure 3 show that Δ
ρrmax from APOD appears at (50–60° S, 80–140° E) at dawn and (70–75° S, ~180° E) at dusk from April to August. The global Δ
ρrmax appears at (65–75° N, 60–120° W) at dawn and at (~75° N, 60–100° W) at dusk from November to February. Comparing the latitudes of Δ
ρrmax at dawn and dusk, the latitudes of Δ
ρrmax at dusk are higher and closer to the geomagnetic pole in the two hemispheres. According to the longitudes of Δ
ρrmax, the positions of Δ
ρrmax at dusk are in the east of that at dawn, especially in the southern hemisphere. The difference between the latitudes where Δ
ρrmax appears at dawn and dusk may be related to the meridional wind.
According to the HL-TWiM empirical model results in the upper panel in
Figure 4, the mid-high latitude thermospheric wind is poleward with a maximum of 76 ms
−1 at 50° S at dusk. At dawn, it is equatorward or poleward with lower values relative to dusk between 50° S and 75° S. Take December as an example. The thermospheric meridional wind between 50° N and 75° N is less than 15 ms
−1 at dawn, weaker than that at dusk.
The largest meridional wind speed reaches above 50 ms
−1, around 60° N at dusk. The more intensive poleward wind might induce the location of Δ
ρrmax extending to the polar region at dusk. In addition, the difference in longitudes where Δ
ρrmax appears at dawn and dusk could be attributed to the zonal wind. From the lower panel of
Figure 4, the zonal wind at the latitude where Δ
ρrmax appears is westward in the two hemispheres at dawn. The westward wind facilitates the westward extension of Δ
ρrmax at dawn. At dusk, the zonal wind at the latitude where Δ
ρrmax appears is eastward in two hemispheres. The eastward wind facilitates the eastward extension in Δ
ρrmax at dusk. The zonal winds could explain the difference in the longitude where Δ
ρrmax appears at dawn and dusk. The zonal wind in the southern hemisphere reaches more than 70 ms
−1, which is more significant than in the Northern Hemisphere. Thus, the difference in the longitude, where Δ
ρrmax appears between dawn and dusk, is pronounced in the Southern Hemisphere. The exact reason may need further study through additional observation and numerical simulation.
Figure 5 shows the seasonal variation of Δ
ρrmax and Δ
ρrmin from APOD at dawn and dusk. The left panel shows that the annual maximum of Δ
ρrmax from APOD occurs in July at both dawn and dusk. Xu et al. [
10] showed that the annual maximum of Δ
ρrmax from the GRACE observations occurred during equinoxes. The difference in peak occurrence time may be due to the different solar activity levels and local times. The results in Xu et al. [
10] are averaged over all local times at high, middle, and low solar activity levels. The results from APOD in the paper are only for around the terminator at a low solar activity level. Xu et al. [
30] and Shreedevit et al. [
31] showed that the seasonal variation of ionospheric density at the high latitudes in the Southern Hemisphere has significant solar activity and local time dependence. Their results showed that the ionospheric density at the high latitudes of the Southern Hemisphere usually has a semiannual anomaly with peaks during the equinoxes for high and middle solar activity conditions, especially during the daytime. The larger ionospheric density may produce larger conductivity and Joule heating during the equinoxes. This could cause GRACE’s largest longitudinal variations of thermospheric density to occur during the equinoxes. It has been reported that the ionospheric density at the high latitudes in the Southern Hemisphere has no significant semiannual anomaly and has a relatively low value during the equinoxes under low solar activity conditions (e.g., [
30,
31]). Thus, the Δ
ρrmax from APOD in this paper has no significant peaks during the equinoxes.
The left panel in
Figure 5 shows that the annual maximum of Δ
ρrmax from APOD reach 26.3% and 39.6% at dawn and dusk, respectively, in July, much greater than the annual maximum of Δ
ρrmax at 480 km from GRACE value 15.2% [
10]. The results from the GRACE observation in Xu et al. [
10] are for all local times. According to the above results at dawn and dusk from APOD (see
Figure 2 and
Figure 3), there are significant differences in the peak locations for different local times. So, the maxima of averaged Δ
ρr for the two local times could be less than Δ
ρrmax at dawn or dusk. Suppose all the observations at both dawn and dusk from APOD are used together to obtain the mean relative longitude variation around the terminator. In that case, the maximum will be ~24%, which is closer to the maximum from the GRACE data. So, the various locations of Δ
ρrmax at different local times could bring the lower values of Δ
ρrmax averaged for all local times. Furthermore, the data observed from 2017 to 2018 in 5° latitude × 1 month bins are used in this work, while the data from 2003 to 2008 in 10° latitude × 2 month bins were used by Xu et al. [
10]. The different time windows and bins can also contribute to the different results. In addition, the difference in Δ
ρrmax may also be due to the different solar activity levels in different years.
The left panel in
Figure 5 shows that Δ
ρrmax from APOD at dusk is significantly greater than at dawn in most months. For example, Δ
ρrmax in June reaches 23.0% and 38.7% at dawn and at dusk, respectively. The difference in Δ
ρrmax between dawn and dusk may be related to their different latitudes. From above, we know that Δ
ρrmax is located at a higher latitude at dusk than at dawn around the solstices. Since a degree in longitude at a higher latitude represents a shorter length, the area of a higher-density region should be larger at dawn than at dusk. The larger area of the higher-density region should contribute to the lower Δ
ρrmax at dawn. The observations from CHAMP [
32] also show that the high-latitude density response is less significant around the dawn sector in both hemispheres.
The right panel in
Figure 5 shows that Δ
ρrmin from April to August is larger than in the other months. The annual minima of Δ
ρrmin from APOD reach −25.6% and −40.1% at dawn and dusk. Δ
ρrmin from APOD at dusk is significantly less than at dawn in most months. For example, Δ
ρrmax in June reaches −23.6% and −37.9% at dawn and at dusk, respectively.
Figure 6 and
Figure 7 show the longitude variations of thermospheric density (Δ
ρr) from the MSIS 2.0 model at 460 km at dawn and dusk, respectively. Similar to the APOD data, the MSIS 2.0 predictions exhibit one zonal peak near the geomagnetic pole in the Northern and Southern Hemispheres. The annual maxima of Δ
ρrmax in the Southern Hemisphere at dawn and dusk from MSIS 2.0 appear in August with values of 28.8% and 34.7%, respectively. The annual maximum of Δ
ρrmax in the Northern Hemisphere from MSIS 2.0 occurs between December and February at dawn and dusk, and both values are ~14%. They are slightly less than the annual maxima from APOD.
Figure 6 and
Figure 7 show that Δ
ρrmax from MSIS 2.0 in the Southern Hemisphere is larger than those in the Northern Hemisphere at dawn and dusk from March to September. The MSIS annual maximum of Δ
ρrmax appears in the Southern Hemisphere, as the results from APOD show. There are some differences between Δ
ρrmax from APOD and MSIS 2.0. From November to February, Δ
ρrmax from MSIS 2.0 in the Southern Hemisphere is larger, while Δ
ρrmax from APOD in the Northern Hemisphere is larger. Take December as an example. In December, Δ
ρrmax in the Northern Hemisphere from APOD is 13.2% and 25.4% at dawn and dusk, respectively. Δ
ρrmax from MSIS 2.0 is only 9.0% and 14.3%, respectively. In the Southern Hemisphere, Δ
ρrmax from APOD in December is only 9.0% and 7.1% at dawn and dusk, respectively. Δ
ρrmax from the MSIS 2.0 model is 13.4% and 18.8%, respectively. Thus, Δ
ρrmax from MSIS 2.0 appears in the Southern Hemisphere in all months. Correspondingly, Δ
ρrmax from APOD appears in the Southern Hemisphere near the equinoxes and in the winter hemisphere around the solstices. The MSIS 2.0 model might overestimate the longitudinal variations of thermospheric density in the Southern Hemisphere and underestimate them in the Northern Hemisphere around the December solstice.
As described above, the comparison of the APOD density between dusk and dawn indicates that Δ
ρrmax at dusk from APOD is located at a higher latitude with larger values than at dawn. It can be seen in
Figure 6 and
Figure 7 that the MSIS 2.0 results have the same characteristics. Δ
ρrmax from MSIS 2.0 is located at 45–50° S and 45–60° N at dawn and located at 70–75° S and 60–75° N at dusk. It is the same as the observations in two hemispheres from APOD that Δ
ρrmax from MSIS at dusk is located at a higher latitude than at dawn. In addition, it can be seen that the value of Δ
ρrmax from MSIS 2.0 is also more pronounced at dusk than at dawn, similar to the result from APOD. For example, Δ
ρrmax from the MSIS 2.0 model in August is 28.8% and 34.7% at dawn and dusk, respectively.
The thermospheric mass density is the product of the average molecular weight and number density. The thermospheric composition and number density also have complicated spatial and temporal variations [
33]. To further analyze the cause of the longitudinal distribution of the thermospheric density, the average molecular weight and number density of the atmosphere at 460 km from the MSIS model are calculated. The relative longitudinal distribution of the average molecular weight and number density versus the zonal mean values at dawn and dusk are given, as shown in
Figure 8,
Figure 9,
Figure 10 and
Figure 11, respectively.
Figure 8,
Figure 9,
Figure 10 and
Figure 11 demonstrate both the atmospheric average molecular weight and number density maxima near the geomagnetic pole in the Northern or Southern Hemispheres. The maximum atmospheric average molecular weight appears in the winter hemisphere during the solstices, and the maximum atmospheric number density appears in the Southern Hemisphere. So Δ
ρrmax from MSIS 2.0 is located in the winter hemisphere during the solstices, more pronounced in the Southern Hemisphere than in the Northern Hemisphere. In some sense, the longitudinal variation of Δ
ρrmax from APOD should also be related to the distribution of the atmospheric average molecular weight from the MSIS model, as the average molecular weight from the model is used in the inversion of the atmospheric density [
16]. More observational data are needed to verify the current study further.
Figure 12 and
Figure 13 show the longitudinal distribution of thermospheric temperature at 460 km from the MSIS model. There is a significantly high-temperature region around the geomagnetic pole at each hemisphere every month. The global maximum occurs in the southern hemisphere. The longitudinal distribution of the MSIS temperature is close to that of the MSIS density. The temperature denotes the thermal energy in the thermosphere. Thus, the longitudinal distribution of density could be related to the thermal energy and the atmospheric temperature. The high density and temperature region around the geomagnetic pole could be related to the aurora heating.