A Possible Distribution of Ion Density in the Iono-Exosphere with a Dipole Magnetic Field

Abstract

The base of the iono-exosphere is defined as the level where the mean free path of ionized particles in a horizontal direction equals the scale height for the ionized gas. It is assumed that an ionized particle moving upward from this base level spirals up along a line of magnetic force without suffering any collision until it returns back to the starting base or goes across the equatorial plane and reaches its conjugate base on the other hemisphere. Due to external forces such as the gravitation, an ion with low velocity at the base can not pass across the equatorial plane but turns back at a certain point on the line of force. The critical velocity at the base for an ion which turns back at any given point on a line of force is calculated as a function of pitch angle and the magnetic latitude assuming a dipole magnetic field. Thus, ionized particles with velocities exceeding the critical velocity at the base contribute to the ion density at farther points. The computed result shows a steeper vertical gradient of the ion density at the equator than at a high latitude. The ion density at 5 earth radii in the equatorial plane is estimated to be about 9 ions cm-3.