MODULATION OF INTERPLANETARY HYDROGEN DENSITY DISTRIBUTION DURING THESOLAR-CYCLE

A new, time-dependent model including variation of the solar radiation pressure and ionisation rate due to solar cycle-related effects is de veloped for a more realistic description of the density distribution o f the hot (similar to 8000 K) hydrogen gas of interstellar origin in t he interplanetary space. It is shown that these temporal variabilities induce significant modulation of the hydrogen density, especially pro nounced in the inner Solar System. The resulting density waves may pro pagate throughout a large part of the heliosphere, before being practi cally damped out beyond similar to 20 AU in the upwind direction and s imilar to 60 AU in the downwind region, correspondingly. The solar cyc le-related density variations lead to periodic ''breathing'' of hydrog en cavity, whose size varies by similar to 15% around the mean value, reaching its maximum size during solar minimum conditions. The effect of the hydrogen density variations on the location of the maximum emis sivity region (MER) contributing to the interplanetary Lyman-alpha glo w is briefly discussed. It is also shown that the mentioned solar cycl e effects induce variation of the intensity of the interplanetary Lyma n-alpha glow as seen from the inner Solar System not only due to the m odulation of the solar Lyman-alpha illumination but also via induced t emporal modifications of the hydrogen density distribution. However, a t large heliocentric distances the long-term solar variabilities affec t the backscattered Lyman-alpha intensity practically only due to sola r Lyman-alpha instantaneous illumination variations. Therefore possibl e discrepancies between the features observed by deep space probes and inferred from the conventional ''hot'' model cannot be ascribed to th e neglect of the solar cycle effects in modelling of the hydrogen dens ity distribution.