MODELING OF THE INTERSTELLAR HYDROGEN DISTRIBUTION IN THE HELIOSPHERE

The detailed knowledge of the distribution of neutral interstellar hyd rogen in the interplanetary space is necessary for a reliable interpre tation of optical and H+ pickup ions observations. In the paper, we re view the status of the modelling efforts with the emphasis an recent i mprovements in that field. We discuss in particular the role of the no nstationary, solar cycle-related effects and the consequences of hydro gen filtration through the heliospheric interface region for its distr ibution in the inner Solar System. We demonstrate also that the use of the simple 'cold' model, neglecting the thermal character of the hydr ogen gas (T similar to 8000 K), is generally incorrect for the whole r egion of the inner heliosphere (R < 5 AU) since it leads to a substant ial underestimation of the local hydrogen density and thus influences the derivation of the H properties in the outer heliosphere/LISM. Refe rring to recent Ulysses measurements, we point out also the need to co nsider in the modelling the effects of the latitudinal asymmetry of th e ionization rate.