ON THE IMPORTANCE OF THE STOCHASTIC CHARACTER OF RADIATION PRESSURE IN EXOSPHERIC MODELING

The influence of the stochastic character of Lyman-a radiation pressur e upon hydrogen atoms in the upper geocorona is studied on the basis o f a Monte Carlo modelling of atomic trajectories. The right way of cal culating mean (bound) orbits is by averaging the positions of atoms ta ken for given (preselected) osculating orbital phases, because simple averaging gives erroneous results. Density distributions created by be ams of atoms launched under identical initial conditions in the middle and upper exosphere are appreciably diluted in comparison with ''cris p'' patterns given by atoms tracked in the ('classic') continuous radi ation pressure case. Particular care is needed when modelling the uppe r geocorona, where the escape rate of atoms may need recalculation due to the stochastic character of radiation pressure: in some cases part icles launched under identical initial conditions may leave the geocor ona, while others reenter the exobase. The need to use a Monte Carlo s cheme in exospheric modelling is discussed. A simple criterion of empl oying the stochastic scheme to track exospheric atoms - to account for the stochastic character of the radiation pressure in Monte Carlo exo spheric modelling - is offered.