A quantitative vector representation of the magnetospheric field was deveoped specifically for the complex event of July 29, 1977. The magnetic field associated with each of the three major magnetospheric current systems was 'controlled' by solar wind and interplanetary field parameters and a magnetic index. This dynamic field model was compared with observed field data and found to be in good agreement. The procedures developed for quantitatively modeling time varying magnetospheric phenomena are general, and eventually it should be possible to use them to 'predict' some aspects of magnetospheric behavior.