CUSP CURRENT MODELING - A SYSTEMATIC-APPROACH

We describe the achievement of a significant step in a program whose u ltimate goal is a self-consistent model that matches cusp currents fro m both ionospheric and magnetospheric sources. Building on existing mo dels, we calculate currents that arise from the ionosphere's ohmic res ponse to the interplanetary electric field mapped to the polar cap. Un like other models, the mapping is confined to the vicinity of the cusp , simulating limited direct communication with the solar wind, as mand ated by recent observations. The mapped area is then inserted into the dayside gap of a purely ionospheric expanding polar cap model. The re sulting two-cell convection patterns have a pronounced kink in the ant isunward flow contours at the poleward boundary of the cusp area, wher e it joins the expanding polar cap. A similar kink appears in the Hepp ner-Maynard empirical patterns; the model implies that it marks the bo undary where the ionosphere takes control from the solar wind. The mod el serves as a first tool for identifying Birkeland current types and causes. Three types arise: (1) Currents border the open cusp like regi on 1 currents bordering a steady state, mini-polar cap. (2) Within the cusp, currents arise from curvature of the equipotentials which, when mapped to the magnetopause, correspond to flow directed away from the merging line, representing merging outflow. (3) The usual region 1 cu rrents line the polar cap boundary along the flow reversals of the two convection cells as a result of polar cap expansion; they are the ion ospheric response to the applied electric field. Thus the model predic ts two types of region 1 currents, driven directly (1) and indirectly (3) by the solar wind, plus a set of pure cusp currents (2). Together they form the familiar overlapping pattern in the cusp region deduced from observations.