Diffusive equilibrium in the topside ionosphere

Diffusive equilibrium is an idealized concept, representing a limiting case of the continuity equation for charged particles in the ionosphere. Since plasma diffusion can proceed rather rapidly above the F
2
peak, the plasma density distribution in the topside ionosphere can, in general, be approximated by a diffusive equilibrium distribution. The density scale height of this distribution depends on the mean ionic mass (i.e., ion composition), and the electron and ion temperatures and their gradients. While diffusive equilibrium is reasonably representative of the distribution of plasma density (and therefore of the major ions), the distribution of the minor ions in the topside ionosphere may still be affected strongly by chemical and transport processes. Because of the magnetic field control of plasma diffusion, equilibrium conditions apply along magnetic field lines. At latitudes where the magnetic dip is fairly high (I > 60°), diffusive equilibrium applies directly to vertical density profiles. At high latitudes (beyond the "trough"), where the local plasma density can be very low, an almost "collisionless" regime prevails, leading to an "ion-exosphere" distribution, which decays with altitude much faster than a diffusive equilibrium distribution. Although diffusive equilibrium may not always be strictly applicable, the concept provides a readily available analytical model for the general features of the topside ionosphere.