Effects induced by the passage of the Sun through dense molecular clouds - I. Flow outside of the compressed heliosphere

The problem of passages of the Solar System through dense, dusty and cold interstellar gas clouds is discussed. A considerable number of such encounters must have already occurred in the past and these events must have pushed the heliosphere inward to the region of the terrestrial planets, provided a number density of the cloud's material of the order of, or greater than, 1000 cm(-3), and relative velocities of about 20 km s(-1) prevail. Under these conditions it will happen that the Earth is directly exposed to the interstellar flow. Neutral interstellar hydrogen and proton densities at the Earth's orbit are calculated under conditions of the heliosphere with the heliopause reduced to dimensions smaller than 1 AU. A simple two-fluids treatment of the incoming flow, ionized by the solar UV only, is proposed here and the amount of neutral interstellar hydrogen is assessed which is accreted by the Earth's atmosphere during one passage. The behavior of the flow variables is investigated by a 2D-hydrodynamic approach to model the interaction processes. Because of a reduction of the heliospheric dimension down to 1 AU, direct influence of the cloud's matter to the terrestrial environment and atmospheric chemistry must be envisaged. This will be briefly discussed in the paper.