# Longitudinal Structure in the Altitude of the Sporadic E Observed by COSMIC in Low-Latitudes
## 4 Discussion
Additionally, the similar conclusion is expected in the Southern hemisphere, because the phase there shows a similar distribution in the altitude range of ~95-125 km, below which the effect is not important as the ion layers seldom descend down there. Figure 9 shows the distributions of the amplitudes for the DE3 tide in the zonal wind during different seasons. Similarly, the amplitudes are provided by the CTMT model. As shown, the largest amplitude primarily occurs in the altitude range of 95-115 km, where most Es layers occur. As a result, the WN4 structure becomes a prominent feature in the Es layer. The DE3 tide shows the obvious seasonal variation, with the largest amplitude in September and smallest one in December. This tide is stronger in June than in March. The seasonal variation agrees well with that of the WN4 structure for the Es height, except for the relative weak WN4 at the June solstice in the Southern hemisphere due to the low occurrence rate of the Es layer there. This low occurrence rate is attributed to the interhemispheric transport of metallic ions by the thermospheric meridional circulation [36]. In March, the DE3 is stronger in the Southern hemisphere than in the Northern hemisphere, which agrees well with the asymmetrical distribution of the WN4 structure as shown by Figure 7. The situation changes in September as the amplitude of the DE3 tide generally shows symmetrical distribution, with the peak located slightly southward of the equator  
Figure 8: (**a**) The phase distribution of the DE3 tide for the zonal wind in September; (**b**) the altitude distribution of the convergence null for the DE3 tide in the zonal wind in September. by \(\sim\)5\({}^{\circ}\). However, the WN4 structure shows a contrary behavior with larger amplitude in the Northern hemisphere. This different behavior needs to be further investigated. Another interesting feature presented by Figure 9 is the latitudinal distributions during June and September. It was found that the peak amplitude of the DE3 tide occurs in the equatorial region at those times. However, as shown by Figure 7, the strongest WN4 structure occurs in low latitudes but not above the equator, especially for the Northern hemisphere where the strongest WN4 structure is located at \(\sim\)15\({}^{\circ}\) N. This is due to the effect of the configuration of the geomagnetic field, which is another key parameter that controls the Es formation, except the wind. The windshear mechanism fails to work near the magnetic equator, where the geomagnetic field is horizontal, because the ions cannot converge into a layer easily due to the Coulomb force induced by the strongly magnetized electrons. Instead, they are kept near a fixed magnetic field line to maintain plasma neutrality. The windshear mechanism becomes increasingly effective as the geomagnetic inclination angle increases from the equatorial region to the low latitudes. However, the amplitude of the DE3 tide decreases at the same time. Thus, the strongest WN4 structure occurs in the low latitudes. As shown in Figure 2, the WN4 structure also occurs above the magnetic equator, where the Es layer is primarily driven by the electric field [51]. The WN4 there is possibly related to the WN4 structure of the electric field [52]. According to the windshear theory, the electrons are Coulomb-forced to follow the ions in the ion convergence process, moving along the geomagnetic field lines. However, the tides in the neutral wind propagate along the geographic latitude. Thus, the difference between the geomagnetic and geographic latitude may have some influences on the tidal signature in the Es layer. We assume that the inconsistency of the eastward movement for the WN4, as shown by Figure 6 in the Southern hemisphere, is related to the difference between the geomagnetic and geographic latitude. This is due to the offset between the invariant magnetic and the geographic poles being larger in the Southern hemisphere than in the Northern hemisphere [53]. However, this view is incomplete and merits further investigation.