# Longitudinal Structure in the Altitude of the Sporadic E Observed by COSMIC in Low-Latitudes
## 2 Data  
GNSS RO is a widely used remote sensing method to monitor ionospheric weather. The amplitude and phase of the GNSS radio signals are usually used to derive the ionospheric scintillation index, which can represent the occurrence of ionospheric irregularities. The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) is one of the satellite constellations dedicated to RO missions, which consists of six low earth orbiter micro-satellites [31].  
In this work, the scintillation (S4) index provided by the COSMIC RO is employed to identify the Es layer during 2007-2018. The data are available on the COSMIC data analysis and archive center (CDAAC) website ([[http://cdaac-www.cosmic.ucar.edu/](http://cdaac-www.cosmic.ucar.edu/)]([http://cdaac-www.cosmic.ucar.edu/](http://cdaac-www.cosmic.ucar.edu/)) (accessed on 1 September 2021)). The precise orbit determination (POD) antennas onboard sample the amplitude at the L1 band with a 50 Hz rate and calculate the standard deviation with a 1 Hz rate. Then the S4 index is reconstructed by CDAAC ground processing after these data are downloaded. The procedure of deriving the S4 index can be found in the report of [PERSON] et al. [32]. To avoid the potential effect of geomagnetic disturbances, S4 index under geomagnetic quiet conditions (Kp \(\leq\) 3) are used in the following analysis.  
The approach to identify Es layers from the profiles here is the same as that in the report of [PERSON] et al. [29]. An RO profile with a maximum S4 index larger than 0.3 in the height range of 85-125 km could be considered to contain an Es layer. The altitude of the maximum S4 index in the profile is assumed to be the peak height of the Es layer. The height interval is set to 85-125 km because Es layers can hardly be formed below 85 km due to the high ion-neutral collision frequency [7; 8] and these layers can appear up to 125 km [5]. This approach has been proven effective by [PERSON] et al. [33; 34] and has also been used by [PERSON] et al. [35; 36] and [PERSON] et al. [37]. [PERSON] and [PERSON] [38] reported a high correlation for Es heights obtained by the S4 index and ionosonde measurements. Thus, although the Es altitude derived from the S4 index here might deviate slightly from the correct height [38], these differences should not influence the general features of the climatology, especially for the longitudinal variations focused on in this paper.  
Based on the COSMIC S4 data, a large database is obtained for the analysis of the global behavior of the Es altitude. Figure 1 shows the altitude distribution of Es layers derived from the COSMIC S4 index over the low latitudes in both hemispheres during 2007-2018. The Lat range is taken from 30\({}^{\circ}\) S to 30\({}^{\circ}\) N. It can be seen from Figure 1 that most Es layers occur in the height range of 90-115 km, with a maximum number at \(\sim\)100 km. The peak at \(\sim\)100 km is due to the combined effects of the altitudinal variations for the metal ions density [5] and the ion-neutral collision frequency [7]. The altitude distribution of the Es layer here generally corresponds well with those from other studies [5; 35].