# Longitudinal Structure in the Altitude of the Sporadic E Observed by COSMIC in Low-Latitudes
## 1 Introduction  
Sporadic E (Es) layers are thin-layered structures with intense, high electron densities at \(\sim\)80-120 km altitudes. They can become denser than the normal E layer or even the peak F layer. The large electron density structures and their strong vertical density gradients may seriously affect the radio propagation and navigation systems [1]. Traditionally, Es layers are detected by ground-based instruments including ionosondes, incoherent and coherent scatter radars, and in-situ probes onboard rockets. In the past decades, global navigation satellite system (GNSS) radio occultation (RO) signals have been widely used to investigate Es layers [2, 3, 4, 5, 6]. It is widely accepted that the mechanism responsible for the formation of this layer is the wind shear at mid and low latitudes. Rocket and incoherent scatter radar observations have shown that Es layers are caused by the vertical convergence of metallic (monoatomic) ions, such as Fe\({}^{+}\), Mg\({}^{+}\), and Na\({}^{+}\)[7]. As a result, these layers are also reported to be related to the sporadic metal layers [8, 9, 10]. Numerous studies have investigated the latitude, altitude, thickness, local time (LT), and seasonal and hemisphere variations of the Es layer and their relations to the wind shear, geomagnetic dip angle, meteor influx, and the tidal and planetary waves [11, 12, 13].  
Since the neutral winds are dominated by solar tides in the E region [14], the close connection between tides and the Es layer is to be expected. The diurnal, semidiurnal, terdiurnal, and quarterdiurnal signatures in the Es intensity and occurrence rate have been reported by previous papers [15, 16, 17]. The relationship is particularly close between tides and the Es altitude, since the Es layer usually descends following the wind shear convergence null (where the ion drift velocity divergence has minimal negative value), which is mainly formed by the tidal wind. Based on measurements from the incoherentscatter radar (ISR), [PERSON] [18] showed that diurnal and semidiurnal tides were the main agents that control altitude descent of Es layers. This view was also supported by results in ionogram data [19]. [PERSON] et al. [17] analyzed the ISR data from Arecibo and found that the descent of Es layers was controlled by diurnal tides at low altitudes and semidiurnal tides at higher altitudes. In addition to diurnal and semidiurnal tides, the terdiurnal tidal signature can also be seen in the height variation of the Es layer [5; 15]. Generally, the non-migrating tide, which is generated by the latent heat release in the troposphere and propagating upward [20], can induce the longitudinal structure in the thermospheric and ionospheric parameters [21; 22; 23; 24; 25; 26; 27; 28]. However, none of previous works has investigated the non-migrating tidal signature of the Es altitude.  
Recently, the WN4 pattern has been observed in the occurrence rate of Es layers over the mid and low latitudes [29]. [PERSON] [30] found some consistencies and inconsistencies between the WN4 pattern and the DE3 tide. He demonstrated that the zonal ion drift factor effect is one cause for the inconsistencies. [PERSON] et al. [29] reported that the WN4 structure for the occurrence rate of Es layers was connected with the DE3 tide in the zonal wind. However, it is still unclear whether the longitudinal structure occurs in the height of Es layers. Therefore, the purpose of this paper is to investigate the longitudinal structure in the altitude of the Es layer. The data used in this work are introduced in Section 2. The general features of the longitudinal structure are described in Section 3. The correlation between the DE3 tide and the longitudinal structure and the effect of the geomagnetic field are discussed in Section 4. The main conclusions are summarized in Section 5.