Exospheric model of the plasmasphere

ISEE observations indicate that after prolonged quiet periods, the saturate d equatorial density decreases exponentially with the radial distance in th e plasmasphere. No hydrostatic barometric models fit the saturated equatori al density profiles, since those obtained with barometric maxwellian or lor entzian kinetic models decrease more slowly with the radial distance. Orbit s of trapped particles are in complete thermal equilibrium with the escapin g and ballistic ones only when the Coulomb collision time is much smaller t han the inter-hemispheric flight time of the ions and electrons. At large r adial distances and in the outermost flux tubes (i.e. for large L-parameter s), this condition is not always fulfilled. It is shown that hydrostatic ex ospheric models with an unsaturated and L-dependent population of trapped p articles can be used to fit the observed density profiles. Such hydrostatic exospheric models with unsaturated population of trapped particles can the refore be used to construct empirical 3D models of the density distribution in the plasmasphere. At small L and low altitudes, trapped orbits are satu rated, while at large distances, these orbits are almost completely deplete d. The fraction eta (L) of required trapped particles necessary to obtain a good fit to observed equatorial density profiles is a function of L. We ha ve determined this function by fitting our hydrostatic theoretical equatori al electron density profiles to that observed by Carpenter and Anderson( 19 92) from L = 2 to 8 after a prolonged period of quiet conditions. Differenc es between maxwellian and lorentzian hydrostatic barometric models and exos pheric hydrostatic models for different values of the kappa index are prese nted. (C) 2001 Elsevier Science Ltd. All rights reserved.