Rotating stars or planets like Earth are sometimes surrounded by a dipolar magnetic field distribution. The thermal plasma forming a corona or an ionosphere around these astrophysical objects diffuses upward along the magnetic field lines and forms a toroidal region filled with this thermal plasma, like the terrestrial plasmasphere. The field‐aligned distribution of this thermal ionospheric plasma is controlled by the gravitational and pseudocentrifugal potential distribution. One can distinguish two extreme types of plasma distribution in this field‐aligned potential: the diffusive equilibrium distribution and the exospheric equilibrium distribution corresponding, respectively, (i) to a saturated, and (ii) to an almost depleted, magnetic flux tube. As a result of pitch angle scattering by Coulomb collisions, an increasing number of ions escaping from the ionosphere are stored in trapped orbits. These trapped particles have magnetic mirror points at high altitudes, i.e., in the low‐density exospheric region. Also as a result of collisions, the field‐aligned density distributions irreversibly evolve from exospheric equilibrium with a highly anisotropic pitch angle (cigarlike) distribution to a diffusive equilibrium distribution characterized by an isotropic pitch angle distribution. It is shown that the suprathermal ions become anisotropic much more slowly than ions of energies smaller than 1 eV.