Distribution of Density in an Ion-Exosphere of a Nonrotating Planet

A model of an ion-exosphere is set up and an expression is derived for the variation of density with altitude. The ions are considered to be under the control of superimposed gravitational and static, centered-dipole magnetic fields and to have mirror points in the barosphere. Collisions are ignored and the particles are assumed to be trapped in the dipole field. By use of the first adiabatic invariant and Liouville's theorem, the density is found to fall off faster with height in the exomagnetosphere than in either a neutral exosphere or an ionized barosphere.