# Longitudinal Structure in the Altitude of the Sporadic E Observed by COSMIC in Low-Latitudes
## 4 Discussion
It is generally accepted that the vertical divergence of the vertical ion velocity (\(\frac{dw_{z}}{dz}\)) is mainly governed by the vertical divergence of the zonal wind velocity over the mid and low latitudes below \(\sim\)125 km [5; 29; 48; 49]. Therefore, the altitude variation of the Es layer can be represented by the trace of the convergence null, which is defined as the altitude where the tidal wind velocity is zero and the vertical divergence of the tidal wind velocity reaches the minimal negative value (the wind direction is westward above and eastward below this altitude). Figure 8a shows the phase distribution of the DE3 tide in the zonal wind in September. The DE3 tide is provided by the Climatological Tide Model of Thermosphere (CTMT) model. The CTMT is a climatological tidal model, which was developed by [PERSON] et al. [50] based on wind and temperature measurements from the thermosphere-ionosphere-mesosphere-energetics and dynamics (TIMED) satellite. As demonstrated in Figure 8a, the phase of the DE3 tide generally shows symmetrical distribution over the Northern and Southern hemispheres in the altitude range of \(\sim\)95-150 km. In the Northern hemisphere, the altitude range of 80-125 km almost covers a full vertical wavelength of the DE3 tide. Thus, a convergence null will occur in that altitude range at any LT. Figure 8b shows the altitude distribution of the convergence null within that altitude range for the zonal band at 15\({}^{\circ}\) N, where the WN4 is very strong, as shown by Figure 7. Note that the height resolution for the CTMT model is 2.5 km, so the result is unsmooth. It can be found that four maximums and four minimums occur in the zonal band at any LT. The maximums and minimums are separated from each other by \(\sim\)90\({}^{\circ}\) in longitude. They move 90\({}^{\circ}\) eastward in Lon in 24 h. However, the longitudinal variation of the height for the convergence null is not a wave-like structure due to the sharp gradient between the maximums and the minimums. When compared with the result shown in Figure 5, it was found that the distributions of the maximums are in good accordance with those of the crests in Figure 5, while the distributions of the minimums are quite different from those of the troughs. The consistency between the distribution of the crests for Es altitude WN4 and that of the maximum heights for the convergence null supportsthe connection between the WN4 structure and the DE3 tide in the zonal wind. The difference between the troughs in Figure 5 and the minimums in Figure 8b is due to the enhanced ion-neutral collisions at lower altitudes. According to the windshear theory, a layer remains inside a shear convergence null only if it forms rapidly enough compared with the time required for the null to phase-propagate downward a distance equal to the layer's width. The layers form rapidly at higher altitudes, therefore, they tend to \"stick\" at a wind shear convergence null as it descends with the vertical tidal phase speed, while at lower altitudes the situation changes gradually because ion-neutral collisions become increasingly effective in opposing the ion convergence. Therefore, the layers cannot form rapidly enough to remain in a convergence null. As a result, they lag steadily behind the downward propagating convergence null and descend at a velocity smaller than the vertical tidal phase speed. Finally, the layers will begin to disrupt when they are caught up by a divergent tidal node [7]. According to the simulation performed by [PERSON] et al. [17], the diurnal tide can bring an ion layer down to ~90 km. Therefore, it is reasonable that the distribution of troughs for the WN4 structure in Figure 5 is inconsistent with that of the minimums in Figure 8b. We only show the result for the zonal band at 15\({}^{\circ}\) N as an example, since the phase of the DE3 tide shows the minor latitudinal variation in low latitudes.