Simultaneous measurements of the low-altitude energetic particle flux by NOAA spacecraft and the geostationary magnetic field by GOES 2 spacecraft are used to test the recently proposed isotropic boundary algorithm (IBA) method to evaluate the instantaneous magnetospheric configuration. According to the IBA method, the equatorward boundary of the isotropic proton precipitation, in brief the isotropic boundary (IB), corresponds to the boundary separating adiabatic and chaotic regimes of particle motion in the tail current sheet and is controlled by the properties of the equatorial magnetic field. In this study we confirm some of the fundamental features of the IBA method. First, we show that the low-altitude IB position of 30- to 300-keV protons is strongly controlled by the equatorial magnetic field in the tail. (The corresponding correlation coefficient exceeds 0.9.) Second, the MLT dependence of the nightside IB latitude is in good agreement with that computed using magnetospheric models. Third, the observed magnetic field and the field predicted by the IBA method using the measured IB position have similar values and are well correlated with a correlation coefficient of at least 0.84 for the main components and a standard deviation of only about 10% of the dynamic range of these components. This shows that the threshold condition separating the two particle motion regimes is fulfilled in the proximity of the IB field line. We argue that the remaining inconsistencies between the calculated and observed magnetic fields are mainly due to the fact that the available magnetospheric models seem to underestimate the amount of tailward stretching of both the tail field lines during active conditions as well as field lines starting from the dayside. In view of its good capabilities to remotely determine the instantaneous magnetic field, we expect that the IBA method will find wide applications in the mapping of magnetic field lines and in testing of existing and new magnetospheric models. ¿ American Geophysical Union 1993