We have conducted an investigation into the relationship between magnetic field fluctuations below 100 Hz observed with the UARS and Freja magnetic field experiments and concluded that increases in this ac activity serve as an excellent indicator of the boundaries of large-scale field-aligned current systems. Magnetic field fluctuations in these regions generally correspond to increased electron fluxes at energies below roughly 2 keV. Using a single equatorward boundary crossing as a reference point, the statistical field-aligned current pattern of Iijima and Potemra [1978> for disturbed magnetic conditions can be extrapolated to provide an estimate of the global position of the auroral oval. Real-time in situ magnetic field measurements from high-latitude spacecraft can be used to provide a quick, simple, locator of the equatorward boundaries of the large-scale field-aligned currents, accurate to within ¿2¿ in latitude. A proof-of-concept implementation of this technique has been performed using magnetic field data acquired by the Freja spacecraft. Development of the automated detection algorithm, essentially a measurement of the standard deviation of the low-frequency magnetic field measurements, was aided by using UARS magnetic field data. The UARS data were binned by MLT and magnetic latitude for varying Kp conditions and demonstrated observable ac boundaries at all local times and all Kp levels. In a separate statistical study consisting of 96 UARS high-latitude observations, we determined the differences between the observed and estimated equatorward boundaries of the large-scale field-aligned current system for each high-latitude pass. This difference was within 2 deg of latitude in 53 of 96 cases, with 84 of 96 cases within 5 deg. ¿ 1998 American Geophysical Union