NUMERICAL MODELING OF THE RING CURRENT AND PLASMASPHERE

Over the last 25 years, considerable scientific effort has been expend ed in the development of quantitative models of the dynamics of Earth' s inner magnetosphere, particularly on studies of the injection of the storm-time ring current and of dynamic variations in the shape and si ze of the plasmasphere. Nearly all modeling studies of ring-current in jection agree that time-varying magnetospheric convection can produce approximately the ion fluxes that are observed in the storm-time ring current, but the truth of that assumption has never been demonstrated conclusively. It is not clear that the actual variations of convection electric fields are strong enough to explain the observed flux increa ses in similar to 100 keV ions at the peak of the storm-time ring curr ent. Observational comparisons are generally far from tight, primarily due to the paucity of ring-current measurements and to basic limitati ons of single-point observations. Also, most of the theoretical models combine state-of-the-art treatment of some aspects of the problem wit h highly simplified treatment of other aspects. Even the most sophisti cated treatments of the sub-problems include substantial uncertainties , including the following: (i) There is still considerable theoretical and observational uncertainty about the dynamics of the large-scale e lectric fields in the inner magnetosphere; (ii) No one has ever calcul ated a force-balanced, time-dependent magnetic-field model consistent with injection of the storm-time ring current; (iii) The most obvious check on the overall realism of a ring-current injection model would b e to compare its predicted Dst index against observations; however, th eoretical calculations of that index usually employ the Dessler-Parker -Sckopke relation, which was derived from the assumption of a dipole m agnetic field and cannot be applied reliably to conditions where the p lasma pressure significantly distorts the field; (iv) Although loss ra tes by charge exchange and Coulomb scattering can be calculated with r easonable accuracy, it remains unclear whether wave-induced ion precip itation plays an important role in the decay of the ring current. Howe ver, considerable progress could be made in the next few years. Spacec raft that can provide images of large regions of the inner magnetosphe re should eliminate much of the present ambiguity associated with sing le-point measurements. On the theoretical side, it will soon be possib le to construct models that, for the first time, will solve a complete set of large-scale equations for the entire inner magnetosphere. The biggest uncertainty in the calculation of the size and shape of the pl asmasphere lies in the dynamics and structure of the electric field. I t is still not clear how important a role interchange instability play s in determining the shape of the plasmapause or in creating density f ine structure.