# Longitudinal Structure in the Altitude of the Sporadic E Observed by COSMIC in Low-Latitudes
## 3 Results  
Longitudinal structure is such that the height of the Es layer shows zonal difference at a same LT. It can be presented by plotting the global distributions of the averaged Es altitudes in the LT fixed reference. Figure 2 shows such plots for 08-10 LT (the first column), 10-12 LT (the second column), 12-14 LT (the third column), and 14-16 LT (the fourth column) during four seasons denoted by the March equinox (the first row), June solstice (the second row), September equinox (the third row), and the December solstice (the fourth row). Seasons are marked by \(\pm\)45 days of the equinoxes and solstices. The grid resolution is 5\({}^{\circ}\) in geographic longitude (Lon) and 2.5\({}^{\circ}\) in geographic latitude (Lat). The solid line in each subfigure depicts the magnetic equator. The most striking feature in Figure 2 is the wave-like structure, which presents four evident peaks over the low latitudes. This longitudinal structure, which is called the WN4 structure, can be identified in both hemispheres at equinoxes and only in the Northern hemisphere at the June solstice. Despite the different intensities, this structure generally locates symmetrically in both hemispheres at equinoxes. It disappears in both hemispheres at the December solstice and instead, a wave-2 like structure occurs. As shown by Figure 2, the longitudinal structure of the Es height can be identified as the WN4 structure directly from its global distribution during the daytime hours. It is generally located in the low latitudes below 30\({}^{\circ}\). The longitudinal variations of the Es altitude averaged within 0-30\({}^{\circ}\) N are presented in Figure 3 at 2 h local time intervals. The results for the March equinox (black lines), June solstice (red lines), and the September equinox (blue lines) are plotted, since the WN4 structure disappears at the December solstice. Note that different scale labels are used for a better visualization, since the Es altitude shows a local time variation. As shown by Figure 3, the WN4 structure becomes visible during 04-06 LT at equinoxes, which is 4 h earlier than the starting time of its occurrence at the June solstice. The WN4 structure persists during the daytime after its appearance around the dawn until it totally vanishes at 20-22 LT. Generally, the WN4 structure shows the largest height difference between crests and troughs at the September equinox, which can be as large as \(\sim\)6 km at 10-12 LT. The four crests (troughs) are generally separated by \(\sim\)90\({}^{\circ}\) in longitude, and they are located at the similar locations during the three seasons. It is also found that the crests (troughs) move eastward during the daytime. The detailed movement of the crests (troughs) will be described later. Figure 1: The altitude distribution of the Es layer derived from the COSMIC S4 index over the low latitudes during 2007–2018. Similarly, Figure 4 shows the longitudinal variations of the Es altitude averaged within 0-30\({}^{\circ}\) S. It is found that the WN4 structure behaves differently at the June solstice. This structure is evident during 06-14 LT, but it is hard to be identified at other times during the day. This different behavior is possibly due to the low occurrence rate of the Es layer in the Southern hemisphere at the June solstice, which will also make it difficult for us to detect the WN4 structure. It also can be seen from Figure 4 that the duration of the WN4 is generally shorter over the Southern hemisphere than over the Northern hemisphere. The starting times for the occurrence of the WN4 over the Southern hemisphere, which are 06-08 LT for the March equinox (08-10 LT for the September equinox), are later than those over the Northern hemisphere. The WN4 structure disappears at 18-20 LT at equinoxes, which is 2 h earlier than that over the Northern hemisphere. Figure 3: Longitudinal variations of the Es altitude averaged within 0–30\({}^{\circ}\) N at the March equinox (black lines), June solstice (red lines), and the September equinox (blue lines).