## 1 Introduction
Later [PERSON] et al., 2006 [35] and [PERSON] et al., 2006 [42], specified that the diurnal eastward-propagating zonal wave number 3 (DE3) tide is responsible for the generation of the ionospheric WN4 structure. We note that, at a fixed local time, the DE3 wave is seen as four maxima on longitude in all tidal fields. The WN4 structure has also been observed in \(E\times B\) plasma drift [36; 37; 43; 44] and in the equatorial electrojet (EE) [45; 46]. The satellite observations of different ionospheric and thermospheric parameters, such as Total Electron Content (TEC), the electron temperature, F-region zonal neutral wind and neutral mass density, and the ionospheric F2 layer height (_lmr_E2), have also shown WN4 longitudinal structures [26; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56]. The above-mentioned observational evidence illustrating the MLT wave impacts on the ionosphere demonstrated only the ionospheric response without the attendant forcing event from below. [PERSON] and [PERSON], 2010 [26], presented, for the first time, the global structure and seasonal variability of the ionospheric response to the forced-from=below nonmigrating tides DE3 and DE2 (diurnal tide eastward propagating with zonal wavenumber 2), which generate WN4 and WN3 both in the MLT temperature field and in the ionospheric _lmr_E2. Later, by using the FORMOSAT-3/COSMIC (F3/C) electron density profiles and the SABER temperatures, [PERSON] and [PERSON], 2011 [56], utilized them to define the latitude and altitude structures of the ionospheric DE3 and DE2 tidal response to the DE3 and DE2 temperature tides coming from below. An interesting result is the observed two altitude regions of enhanced electron density response: an upper level response maximizing near 400-450 km, and a bottom level one below 250-300 km. Both regions revealed different phase structures, indicating that the upper level response is mainly due to the DE3/DE2-modulated vertical plasma drift, while the bottom level one is complex and caused by more than one mechanism. The next step in studying the ionospheric response to the tidal forcing from below was done by a detailed comparison between the simulated and observed global electron density response to different atmospheric tides forced from below. Such a comparison, reported by [PERSON] et al., 2012 [57], was presented for the first time. For this purpose, the authors used the SABER temperature tidal forcing and the F3/C electron density response for the period of time October 2007 to March 2009, as well as the Earth's whole atmospheric model from the troposphere to the ionosphere, called GAIA [58; 59; 60], for the simulation of the electron density tidal responses. In general, the GAIA model reproduced the F3/C DE3/DE2 responses quite well, particularly the DE3 response. Regarding the altitude structure of the DE3/DE2 response, the simulations indicated that, while the upper level DE3/DE2 response is apparently shaped mainly by the \"fountain effect\", the reason(s) for the lower level is not very clear. It could be a possible direct tidal effect in the considered altitude, mainly due to the vertical wind DE3 tide, or a possible coupling between tidesand the ionosphere via changes in neutral density, and particularly a modification of [O] and mass mixing ration of [O] (measured in kg/kg). All of the above-mentioned studies indicated that the WN4 longitudinal structure is a prominent feature of the low-latitude ionosphere-thermosphere system, and in general it can be reproduced by the models [61; 62; 63]. However, when a given ionospheric longitudinal structure is investigated only at fixed local times (LT), it cannot be directly related to a certain tidal component.