Multiple charge exchange and ionization collisions within the ring current-geocorona-plasmasphere system: Generation of a secondary ring current on inner L shells

摘要

Charge exchange (CE) collisions between ring current ions and geocoronal atoms erode the ring current and give rise to a pervasive flux of energetic neutral atoms (ENAs). ENAs generated in the main ring current traversing the inner magnetosphere car be reionized in several ways, converting ENAs back into ring current ions albeit on new L shells (provided the angle between the direction of ENA travel and the local magnetic field direction at the point of collision corresponds to a pitch angle outside the local loss cone). These displaced ring current ions in turn undergo subsequent CE collisions with geocoronal atoms, generating secondary ENA fluxes that can participate in further ionizing collisions. A modeling scheme for exploring the effectiveness of this mechanism in establishing and maintaining a secondary ring current at L less than or similar to 3 has been developed taking full account of multiple CE and ionizing collisions. It is based on a matrix solution technique for an integral form of the Boltzmann transport equation and relies on two simplifications: (1) Since gyroperiods much less than bounce periods, gyroradii much less than scale lengths for variations in geocoronal and plasmaspheric densities, and single-bounce collisional depths along ion gyropaths much less than 1, it is adequate to work in term of quantities integrated dong bounce paths and averaged in gyrophase. (2) The thermal populations are not directly perturbed by the energetic populations, while the sources of the main ring current are external to the charge-exchange-coupled systems, so that the transport equation for the secondary flux is linear. The results are steady state fluxes of trapped ions as functions of L shell, equatorial pitch angle, and energy. This model is described in full, and an example of steady state secondary H+ ring current fluxes for quiet conditions is presented using the quiet time empirical model of Sheldon and Hamilton 1993 to prescribe the main proton ring current.