IDENTIFICATION OF AURORAL OVAL BOUNDARIES FROM IN-SITU MAGNETIC-FIELDMEASUREMENTS

We have conducted an investigation into the relationship between magne tic field fluctuations below 100 Hz observed with the UARS and Freja m agnetic field experiments and concluded that increases in this ''ac'' activity serve as an excellent indicator of the boundaries of large-sc ale field-aligned current systems. Magnetic field fluctuations in thes e regions generally correspond to increased electron fluxes at energie s below roughly 2 keV. Using a single equatorward boundary crossing as a reference point, the statistical field-aligned current pattern of I ijima and Potemra [1978] for disturbed magnetic conditions can be extr apolated 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 equ atorward boundaries of the large-scale field-aligned currents, accurat e to within +/- 2 degrees in latitude. A proof-of-concept implementati on of this technique has been performed using magnetic field data acqu ired by the Freja spacecraft. Development of the automated detection a lgorithm, essentially a measurement of the standard deviation of the l ow-frequency magnetic field measurements, was aided by using UARS magn etic field data. The UARS data were binned by MLT and magnetic latitud e 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 di fferences between the observed and estimated equatorward boundaries of the large-scale field-aligned current system for each high-altitude p ass. This difference was within 2 deg of latitude in 53 of 96 cases, w ith 84 of 96 cases within 5 deg.