A three‐dimensional numerical model of ionospheric plasma in the magnetosphere

Abstract

The magnetospheric transport of terrestrial plasma is numerically investigated by means of three‐dimensional particle trajectory tracing in empirical models of the geoelectric and geomagnetic fields. Various ionospheric outflows (auroral, polar cap, cusp, and polar wind) are systematically examined using observational definitions of their respective locations and strengths, and assuming purely adiabatic motions under the effect of the large‐scale magnetospheric convection. Due to field model limitations, the simulations are limited in scope to the region within a geocentric radius of 17 RE. Consequently, much of the terrestrial H+ outflow cannot be accurately traced beyond the polar cap region, and the conclusions concerning the terrestrial contribution to plasma sheet H+ are necessarily limited. Many qualitative features of the plasma sheet are produced in the model by the ionospheric plasmas. The motions of terrestrial O+ outflow are well described within the assumptions of the calculation. It is found that, during quiet times, the observed ionospheric outflows provide a negligible contribution of energetic plasma sheet O+, in agreement with plasma sheet observations. In contrast, during high geomagnetic activity, the simulations demonstrate a substantial ionospheric supply of O+ to the energetic plasma sheet, primarily due to the convection of upwelling ions from the cleft ion fountain through the central plasma sheet. These O+ ions experience kiloelectron volt accelerations during such disturbed times and lead to earthward streaming ion distributions along surfaces which may be identified as the plasma sheet boundary layers. They are thus consistent with the observed characteristics of the active plasma sheet. At all levels of geomagnetic activity, the ionosphere yields significant quantities of low‐energy ions (0–100 eV) which provide potential plasma sources for the plasma sheet after nonadiabatic heating processes. Conclusions regarding the quantitative terrestrial H+ contribution to the plasma sheet must await development of field models adequate for trajectory tracing through the distant plasma sheet, and may require inclusion of nonadiabatic effects.