Numerical modeling of the magnetospheric cusp: Ion injection and number density calculations

Abstract

The magnetospheric cusp is the principal site of solar wind plasma entry into the magnetosphere, and plasma entry through this region constitutes an important source of plasma in the Earth's magnetosphere. The goal of this dissertation is to understand the dynamics and location of the plasma injection process and the subsequent transport of this plasma throughout the magnetosphere by numerically modeling the cusp in terms of the "zeroth-order" physical processes. A quantitative model of ion injection and number density in the magnetospheric cusp is developed, incorporating mutually consistent electric and magnetic fields. This work extends the method of Onsager et al., who calculated precipitating particle fluxes from quantitative models of magnetosheath flow and ion acceleration at the magnetopause. We have simulated cusp ion energy-latitude spectrograms at mid-altitude. Both the large-scale energy-latitude dispersion and the embedded small-scale energy-pitch-angle V signatures are clearly evident in these simulated spectrograms. Our results show that a much finer V microsignature is obtained when the ion injection source is restricted to a small region. However, the cutoff of the plasma injection at the magnetosheath sonic line also yields relatively narrow V's, even without restricting the injection region to a small locus on the magnetopause. This effect is most noticeable in winter conditions. To explain the frequently observed multiple cusp ion injections that appear to overlap on the same field lines, we present two independent approaches. Our simulations have successfully reproduced the meso-scale cusp ion overlapping structure by firstly incorporating temporal effects of separate bursts of reconnection which last 1.4 min and are 3.6 and 4.6 mins apart; and secondly by introducing a time-dependent magnetosheath plasma density variation along the magnetopause to our cusp model, even with assuming steady interconnection. Our cusp injection model which returns precipitating particle flux also allows us to calculate the number density profile in the cusp. Our result along the noon-meridian cusp demonstrates that the density gradient is sharper on the equatorward edge than the poleward edge, and that the equatorward edge of the density structure shifts to higher latitude at lower altitude.