A simple model for an ion-exosphere with an open magnetic field is set up. The ions move under the influence of (1) the gravitational field, (2) the monotonic decreasing static magnetic field, and (3) the electrostatic potential due to a small charge separation. Neglecting collisions and particle drift across the magnetic field lines the particles can be classified into four classes: ballistic, escaping, trapped, and incoming particles. For each class the number density, the particle flux, the momentum fluxes, and the energy flux are calculated as a function of the electrostatic potential. Finally, it is shown how this potential can easily be computed by considering two basic physical conditions: (1) the quasineutrality, and (2) the zero current condition.