## 1 Introduction  
Atmospheric tides are caused by solar radiation on the diurnal cycle, by water vapor, and by ozone. As a consequence, temperature and wind are modified due to the absorption of radiation. The composition of the atmosphere (and in particular ozone) is also modified by photo-dissociation. This well-known mechanism, called migrating tides, generates inertia-gravity waves of diurnal and semi-diurnal periods that propagate in the atmosphere synchronized with the rotation of Earth (synchronized with the sun). The basic characteristics of atmospheric tides are described by classical tidal theory. Ideally, tidal theory shows that tides correspond to atmospheric waves that can be viewed as linear perturbations of a zonal, horizontally stratified, isothermal mean state. In fact, atmospheric tides are natural modes of the atmosphere that can be mathematically described by Hough functions having amplitudes increasing exponentially with altitude [1]. While the atmospheric structure is not fully zonally uniform and has daily, monthly, and decadal fluctuations, tide characteristics can be different from theory [2]. Quantifying these fluctuations is a difficult challenge, as frequent observations are needed to sample diurnal cycles. Information about atmospheric tides has been possible through continuous observations over the same site by meteorite radars for wind [3, 4] and temperature [5] in the upper mesosphere and lower thermosphere or by lidar for temperature in the middle atmosphere [6, 7, 8, 9] and lower thermosphere [10]. Fluctuations in water vapor and ozone through photo-dissociation have also been observed with microwave instruments [11, 12]. Atmospheric tides have also been assessed on a global scale from orbiting satellites, either by their twice-daily passes in assuming certain characteristics of the tides (notably their phases), or from instruments that do not have a synchronous orbit with the sun, such as UARS [13, 14] or SABER/TIMED [15]. In this case, successive shifts each day giving different measurement times over the same location cover the whole diurnal cycle in 60 days. However, the large diurnal variability induces spectral aliasing effects especially with planetary waves when the atmosphere is under-sampled. Conversely, quantification of tidal characteristics are biased at mid and high latitudes [16]. Information on tidal characteristics is crucial when comparing measurements that were not made at the same time and, even more, to estimate decadal trends [17] in the middle atmosphere when the orbits of successive satellites have changed [16] or when orbits drift with time [18]. Similar problems arise when data are assimilated to produce re-analyses, if the tidal modeling is not sufficiently realistic and if the tidal characteristics themselves evolve or are modified by background fields due to climate change [2]. Specific global modeling of migrating tides [19] assumes a zonally homogeneous steady atmosphere. Developments associated with the global atmospheric tide model [20] have shown good overall agreement with observations with, however, smaller amplitudes with models compared to observations [21] at mid latitudes. Comparisons with European meteorological analyses, assimilating radiosondes, and operational satellites show semi-diurnal fluctuations consistent with ECMWF (European Centre for Medium-Range Weather Forecasts) with theoretical estimates given by the GSWM model. However, the observed diurnal fluctuations reveal significant geographical differences and sometimes different characteristics of tides, probably due to ground and tropospheric effects. A clearer understanding of tidal changes will be useful for improving methodologies in trend estimates, data assimilation, and local radiative issues. The separation of both types of tides is not trivial; however, the global/regional nature of their extension is proposed to identify both tidal components included in meteorological/climate models that provide global and continuous coverage. Observations are missing for such analyses.