# Longitudinal Structure in the Altitude of the Sporadic E Observed by COSMIC in Low-Latitudes
## 3 Results
Figure 2: Global distributions of the Es height in the LT fixed reference for the March equinox (**the first row**), June solstice (**the second row**), September equinox (**the third row**), and December solstice (**the fourth row**). It was found that the WN4 is the most evident longitudinal structure for the altitude of the Es layer. This structure can be extracted as a component from the measurements by a Fourier transformation (FT) and then an inverse FT. The FT is performed to a series of altitude measurements (\(H_{k}\)) at a longitude series (\(Lon_{k}\) = 5(\(k-1\)), \(k\) = 1, 2... 72) according to the equation: \(A_{W_{n}}=\frac{p}{\sqrt{2}}\cdot\sum\limits_{k=1}^{72}H_{k}\cdot e^{\text{i} \cdot 2\pi\text{m}(k-1)/72}\), where \(p=\left\{ \begin{array}{ll}1,&n=0\\ 2,&n>0\end{array} \right.,\ i=\sqrt{-1}\), \(n=wavenumber\). The calculated \(A_{W_{4}}\) is the WN4 component. Figure 5 shows the WN4 components (the right column) picked out from the Es altitudes (the middle column) averaged within 0-30\({}^{\circ}\)N. The spectral amplitudes \(A_{W_{n}}\) obtained by the FT are shown in the right column. The eastward shift of the crests is clearly seen in the left column at the equinoxes and the June solstice, although the shift of the second crest (from left to right) is not obvious or even slightly westward at the September equinox. As is seen from the right column, the Es altitude consists of different wave components. The WN4 is the major component in the daytime at the June solstice and September equinox. It is also remarkable in the daytime at the March equinox, although the WN1 structure is stronger than it is in the afternoon. At the December solstice, the WN4 is weak and the WN2 is the main component in the daytime. The dashed lines in the middle column denote a longitude of 90\({}^{\circ}\) step within 24 h. The movement of the WN4 component is found to be in general accordance with the dashed line at the equinoxes and the June solstice, which indicates that the average speed of the WN4 pattern is about 90\({}^{\circ}\)/day. Another striking feature in the middle column is the seasonal variation of the WN4. It is strong at the June solstice and the September equinox, and relative weak at the March equinox. The similar results for the FT are shown in Figure 6, which are based on the averaged altitude within 0-30\({}^{\circ}\)S. Generally, the WN4 shows similar phase distributions in the Southern hemisphere to those in the Northern hemisphere. Some different features are shown by Figure 6. Firstly, the WN4 is relatively weaker at the June solstice than at the equinoxes. As shown by the spectrum, the WN1 is stronger than the WN4 in the daytime at the June solstice. Secondly, there is less consistency between the shift of the WN4 and the dashed line than in the Northern hemisphere, which implies the inconsistency of the eastward movement for the WN4. Figure 4: Same as Figure 3, but for 0–30\({}^{\circ}\) S.  
Figure 7 shows the latitudinal variations of the WN4 structure with 5\({}^{\circ}\) intervals within 30\({}^{\circ}\)S-30\({}^{\circ}\)N for the March equinox (the top row), June solstice (the middle row), and September equinox (the bottom row). The intensity of the WN4 is represented by the amplitude (\(A_{W4}\)) derived from the FT. The seasonal variation presented here is similar to that shown in Figure 7. The seasonal variation of the SW4 structure with 5\({}^{\circ}\) intervals within 30\({}^{\circ}\)S-30\({}^{\circ}\)N for the March equinox  
Figures 5 and 6. It was found that the WN4 structure generally shows the largest intensity within 10-20\({}^{\circ}\)N and 5-15\({}^{\circ}\)S in the Northern and Southern hemispheres, respectively. An interesting feature is that the WN4 structure shows asymmetrical intensities in the two hemispheres, although it generally shows symmetrical phases, as shown by Figures 5 and 6.