SIMBAD references

In this first paper, we simulate the population of disc red clump stars to be observed by Gaia. We generate a set of test particles and we evolve it in a 3D barred Milky Way like galactic potential. We assign physical properties of the red clump trace population and a realistic 3D interstellar extinction model. We add Gaia observational constraints and an error model according to the pre-commissioning scientific performance assessments. We present and analyse two mock catalogues, offered to the community, that are an excellent test bed for testing tools being developed for the future scientific exploitation of Gaia data. The first catalogue contains stars up to Gaia G ∼ 20, while the second is the subset containing Gaia radial velocity data with a maximum error of σ_V_r=10 km/s. Here, we present first attempts to characterize the density structure of the Galactic bar in the Gaia space of observables. The Gaia large errors in parallax and the high interstellar extinction in the inner parts of the Galactic disc prevent us to model the bar overdensity. This result suggests the need to combine Gaia and IR data to undertake such studies. We find that IR photometric distances for this Gaia sample allow us to recover the Galactic bar orientation angle with an accuracy of ∼ 5°.