In this paper a useful empirical relationship is established between the readily available magnetic activity index, Kp, and the tail lobe magnetic field strength. A simple (dipole plus tail field plus ring current) magnetic field model is combined with the results of Gussenhoven et al. (1981) that relate the location of the equatorward boundary of auroral oval with Kp. The results are tested by taking over 500 samples of Kp and converting them to the corresponding lobe field strength values which are then plotted as a frequency distribution. This distribution is compared with a similar distribution of over 700 hours of IMP 4 tail lobe magentic field measurements taken during the same period by Fairfield and Ness (1970). The agreement between the two distributions is excellent only when a model ring current is included. The implies that the ring current makes a significant contribution to the location of the equatorward boundary of the auroral oval (Siscoe, 1979), particularly during periods of high Kp. Comparisons are made between the present model and the zero-energy Alfven layer calculations that have appeared in the literature.