INTERPLANETARY NEUTRAL PARTICLE FLUXES INFLUENCING THE EARTHS ATMOSPHERE AND THE TERRESTRIAL ENVIRONMENT

It is well known that the Solar System is swept over by neutral inters tellar gases, primarily hydrogen and helium, entering the heliosphere from the upwind side and penetrating inward, even up to the orbit of t he Earth. The Earth on its orbit is thus moving through this density f ield and is intercepting time-variable hydrogen and helium fluxes. Qua ntitatively, this is associated with a sensitive reaction of the densi ty fields to time-dependent conditions of the solar radiation pressure and the ionizing solar radiations during the solar activity cycle. As we shall show, the density distribution of interstellar hydrogen alon g the orbit of the Earth is strongly varying during the solar cycle. I n connection with the variation of the mean relative velocity of this gas with respect to the orbiting Earth a highly variable hydrogen infl ow into the Earth's atmosphere will be induced. There is also an addit ional source of hydrogen influencing the Earth's environment due to th e fact that neutral interstellar hydrogen and helium are neutralizing solar wind protons by charge exchange inside the orbit of the Earth, t hereby producing an antisolar flux of keV-energetic hydrogen atoms imp inging onto the Earth's atmosphere. These time-variable fluxes could b e directly monitored by gas detectors of an advanced technology. They might also be indicated by indirect terrestrial effects. We are invest igating the question of whether and how these time-variable inflows co uld be recognized by careful studies of relevant upper atmospheric rea ctions. We study particle-induced energy inputs and ionization rates i n the upper atmosphere and analyze influences on the hydrogen geocoron a. We also study the process of relaxation of the inflowing hydrogen w ithin the terrestrial atmosphere and investigate the reaction of atmos pheric hydrogen densities to these variable inflows. While it is shown that the induced short-period variation of upper atmospheric hydrogen densities is of negligible amplitude at the present epoch, long-perio dic variations of the hydrogen inflow like those connected with the en trance of the Solar System into a region of increased interstellar den sity at earlier or later eons clearly give their imprints even at lowe r heights and could even be reflected in ozone depletions induced by i ncreased hydrogen densities at heights of around 50 km. At present ano malous variations of the geocoronal H-1216 A and He-584 A glows uncorr elated with the solar resonance line illumination indicate a breathing of the geocorona under variable interplanetary gas inflows. (C) 1996 Academic Press, Inc.