7.2 The atmosphere The atmosphere is divided by homogeneous sub-layers in order to model the variation of the atmosphere density with the altitude. The density and composition of the layers are computed from the atmospheric model selected by the user such that: i) the atmospheric depths at the altitudes of layer boundaries are equivalent to the depths given by the atmospheric model at these altitudes; ii) the composition and the densities of the layers represent the mean composition and densities given by the atmospheric model in the layers. The thickness and the upper and lower altitudes of each atmospheric layer are computed by respecting the following criteria: i) the detection altitudes defined by the user correspond to layer boundaries; ii) the thickness of all layers are lower than a user defined upper limit and higher than a user defined lower limit; iii) when all the preceding criteria are respected a layer contains a fixed percentage of total atmospheric depth that is defined by the user . The user can define the following parameters for the building of the atmosphere • Maximum and minimum allowed thickness of a layer • The altitude of the top of the atmosphere and of the ground • The maximum percent of atmospheric depth contained in one atmospheric layer • The model that defines the atmospheric composition and density in function of altitude. Following the planet considered the user can select different type of atmospheric models.
 
7.2.2 Earth's atmospheric models Two empirical atmospheric models specific to the Earth case are available in PLANETOCOSMICS: the MSISE90 model and its upgrade version NRLMSISE2000. Both models provide temperature, density and concentration profiles vs altitude from the ground to the exobase (~450-500 km) as function of geographic latitude, longitude, UT, F10.7 index (10.7 cm solar radio flux used as solar UV proxy ), F10.7 A index ( 3month average of F10.7 ) and the geomagnetic index Ap. The dependence of the model on F10.7, F10.7 A, and Ap is negligible below 80 km. For a complete description of these models we refer to [18,19].