New measurements are presented of the L-dependent energy threshold for electron flux isotropy observed near midnight from low-altitude satellites. These data provide the basis for an important remote sensing of the geomagnetic field configurations near the outer edge of the radiation belt. The threshold energy for isotropy observed at low altitudes is interpreted as providing a measure of the effective scale length of inhomogeneity of the geomagnetic field near the equator at high altitude. The data presented were obtained with electron spectrometers on the UARS satellite providing flux and energy spectrum measurements for energies between 30 keV and 5 MeV simultaneously at a variety of pitch angles. On certain orbits the UARS satellite achieved fine L shell resolution and nearly simultaneous measurements at different local times. With this previously unavailable L shell resolution the threshold energy for isotropy versus L shell curves near midnight have shown slopes steeper than 20 MeV per unit L on 20% of the cases, using the International Geomagnetic Reference Field (IGRF) 85 magnetic field model. On 66% of the satellite passes the slopes at the two boundary crossings within 01 to 06 hours in magnetic local time and about 10 min in elapsed time were within a factor of 3 of each other. The L shell at which isotropy occurs above a given threshold energy is found to be generally lower and the slope steeper at times of high geomagnetic disturbance. For the analysis presented here, if isotropy occurs when the radius of field line curvature is less than 10 times the gyroradius of the particles, the scale lengths of inhomogeneities were always found to be less than 2900 km, and in 99% of the cases the scale lengths were greater than 73 km.¿ 1997 American Geophysical Union