THE FIELD LINE TOPOLOGY OF A UNIFORM MAGNETIC-FIELD SUPERPOSED ON THEFIELD OF A DISTRIBUTED RING CURRENT

A magnetic field line topology with nulls, generated by superimposing a uniform magnetic field onto the field from a distributed ring curren t, is analyzed. This simple model, which is reminiscent of the structu res found in laboratory field reversed configurations and detached pla smoids, is amenable to substantial analytical progress and also facili tates the visualization of the three dimensional field geometry. Four nulls are seen to exist and representative field lines and tubes of fl ux found by numerical integration are presented. An infinite number of topologically distinct flux bundles is found. These are distinguished by the number of times they encircle a circular magnetic field line. A convenient mapping is described which proves very useful in distingu ishing between and following the paths of the different tubes of flux as they traverse through the null system. The separatrices that divide these flux bundles are described. The complexities already present in this simple but nontrivial configuration serve to emphasize the diffi culties in analyzing more complicated geometries, but the intuition ga ined from this study proves beneficial in those cases. One such exampl e is the comparison of the generic features of our model with those fo und in a topologically different model of plasmoid formations in the e arth's magnetotail.