The proposed work would apply an existing modular model of the open magnetosphere to three problems that are critical to the NSF Geospace Modeling Program. The objectives of the proposed work are to develop a predictive understanding of (1) injection of solar- wind plasma through the dayside magnetospheric cusp, (2) externally or internally imposed time variations of the polar cap configuration, and (3) reverse convection driven by a nearly northward interplanetary magnetic field (IMF). The proposed method combines physical-numerical modeling with spacecraft data analysis. The model, developed over the past decade with partial support of the GEM program, describes the quantitative mapping of the solar wind electric field from the magnetopause to the polar ionosphere along interconnected magnetic field lines. A machine-potable, user-friendly version of this model for steady-state applications has recently been made available to the GEM research community. A recent extension of this model permits relaxation of the steady- state assumption for time-dependent applications. The first application, modeling of cusp ion injection, involves an ongoing collaboration with T. Onsager to couple his Liouville mapping procedure with the modular open magnetosphere model to produce quantitatively realistic simulations of cusp ion distributions for comparison with real spacecraft data for a general range of input solar-wind/IMF conditions. This study is intended to elucidate the mechanism(s) if injection of solar-wind plasma at the dayside magnetopause to form the cusp and adjacent boundary features. The second application will explore two types of quasi-static time evolution of the polar cap, those driven externally by solar- wind/IMF variations and those driven internally by magnetotail dynamics as communicated to the ionosphere by the electric field mapped from the tail merging line. This study will exploit the unique properties of the polar cap not only as an obs ervation site but also as a reference frame for theoretical-numerical modeling. The third application will continue a collaboration with N.U. Crooker to develop a quantitative model of the overdraped lobe configuration proposed by her to explain the otherwise enigmatic observation of sunward convection on putatively open lobe field lines connected to the solar wind during intervals of nearly northward IMF. This and related northward-IMF phenomena represent perhaps the widest gap in present theoretical understanding of solar-wind-magnetosphere coupling.
