Distribution of Density in a Planetary Exosphere. II

Abstract

In an isolated neutral exosphere three components can be distinguished: (1) elliptic‐ballistic; (2) hyperbolic‐ballistic; (3) bound‐elliptic. Component (2) forms the escape flux which cannot be defined simply. The velocity distribution of all components is calculated as a function of altitude in the exosphere; one can thereby assess the importance of the bound orbits. Even with the bound orbits completely filled, according to detailed balancing, the velocity distribution is non‐Maxwellian; hence the barometric formula cannot be applied to determine the distribution of density. The absence of a Maxwellian distribution is shown to exist at all levels above the reference level (base of the exosphere) and the effective kinetic temperature of the gas is shown to decrease with altitude. The ``base of the exosphere'' forms the dividing level between the barosphere and exosphere; its conventional definition has been re‐examined and it is redefined to correspond to the level from which one‐half of the escaping molecules may escape without suffering any collisions. Numerical results for all components versus altitude are given for neutral hydrogen and oxygen, applicable to the terrestrial exosphere. The contribution of bound orbits is estimated and shown to be minor at all levels. Bound orbits may be important for the exospheres of the outer planets. The interplanetary gas may be considered as an extension of the solar atmosphere; however caution must be exercised in treating it as an exosphere.