Experimental test of magnetospheric models

The barium cloud injection deep into the magnetosphere of September 21, 1971, has revealed that the quiet-time near-equatorial nightside magnetic field is significantly different from that predicted by existing quite-time models. The influence of currents flowing on or near the geomagnetic equator gives the field a more taillike structure at a distance as close as 5 RE. Several models have been tried out to obtain a best fit to the experimental observations of the shape of the field-aligned ion cloud. Although this method does not lead to a unique solution, the most acceptable option emerging from this analysis is a quiet-time tilted image-dipole model with a Williams-Mead tail field and a disclike ring current on the geomagnetic equator. Best-fit ring current parameters are radial extension from 6 to 8.25 RE total current of 1.8 × 106 A. With this ring current, the cloud field lines intersect the 100-km level in the northern hemisphere about 1.5° south of the trace computed for a conventional quiet-time model. Changes in the local field magnitude produced by this current configuration are not inconsistent with quiet-time satellite survey data (Sugiura, 1972), and the model-predicted Dst value is close to the observed value (Sugiura and Poros, 1972).