# Longitudinal Structure in the Altitude of the Sporadic E Observed by COSMIC in Low-Latitudes
## 5 Conclusions  
In this paper, the longitudinal structure in the altitude of the Es layer is investigated for the first time based on the S4 index provided by the COSMIC satellite constellation during 2007--2018. The longitudinal structure is identified as a WN4 pattern within 30\({}^{\circ}\)S-30\({}^{\circ}\)N. The WN4 primarily occurs at the June solstice and at the equinoxes, with the largest amplitude at the September equinox and the smallest one at the March equinox. It persists during the daytime after its appearance around the dawn until it totally vanishes at \(\sim\)20 LT. The duration of this structure is generally shorter over the Southern hemisphere than over the Northern hemisphere. Generally, this structure shows the symmetrical locations over the two hemispheres with an eastward speed of \(\sim\)90\({}^{\circ}\)/day. The strongest WN4 appears within 10-20\({}^{\circ}\)N and 5-15\({}^{\circ}\)S in the Northern and Southern hemispheres, respectively. At the June solstice and the September equinox, the WN4 is stronger in the Northern hemisphere than in the Southern hemisphere, while the situation is reversed at the March equinox. The altitude distribution of the convergence null, which is derived from the DE3 tide in the zonal wind based on the CTMT model, is similar to that of the WN4. This and other similar features, such as the seasonal variation, eastward speed, and the symmetrical locations, support the dominant role of the DE3 tide in the zonal wind for the formation of the WN4. This work demonstrates more comprehensively the effect of the DE3 tide on Es layers and could improve our understanding for the formation and variation of these layers.  
Conceptualization, Z.L.; methodology, Z.L.; investigation, Z.L.; resources, Z.L.; data curation, Z.L. and Q.L.; writing--original draft preparation, Z.L., Q.L., H.F., and Z.G.; writing--review and editing, Z.L., H.F., and Z.G.; visualization, Z.L.; supervision, H.F. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The COSMIC S4 index is available at [[http://cdaac-www.cosmic.ucar.edu/cdaac](http://cdaac-www.cosmic.ucar.edu/cdaac)]([http://cdaac-www.cosmic.ucar.edu/cdaac](http://cdaac-www.cosmic.ucar.edu/cdaac)). The Kp data are available at [[https://www.spaceweather.gc.ca/solarflux/sx-5-en.php](https://www.spaceweather.gc.ca/solarflux/sx-5-en.php)]([https://www.spaceweather.gc.ca/solarflux/sx-5-en.php](https://www.spaceweather.gc.ca/solarflux/sx-5-en.php)). The CTMT model can be obtained at [[http://globaldynamics.sites.clemson.edu/articles/ctmt.html](http://globaldynamics.sites.clemson.edu/articles/ctmt.html)]([http://globaldynamics.sites.clemson.edu/articles/ctmt.html](http://globaldynamics.sites.clemson.edu/articles/ctmt.html)). All accessed on 1 September 2021.  
**Acknowledgments:** We acknowledge the University Corporation for Atmospheric Research (UCAR) for providing the COSMIC data. We acknowledge [PERSON] for providing the CTMT model. The Natural Resources Canada is also acknowledged for providing the Kp data.  
**Conflicts of Interest:** The authors declare no conflict of interest.