Magnetic field measurements made in the Earth's tail region during the growth phases of substorms on May 23, 1979, by GEOS 2 and ISEE 1, and on June 23, 1979, by GEOS 2 and IMP-J, were used to model the magnetic configuration in the location conjugate to the Scandinavian riometer network. Toward the end of the growth phase the time variation of the magnetic configuration explains about 80% of the equatorward expansion of the central part of the auroral oval. Assuming the origin of the observed equatorward moving band of energetic electron precipitation into the ionosphere to be caused by pitch angle scattering in the equatorial current sheet, we inferred the half thickness of the current sheet at r≈9 RE as thin as 0.1 RE near the end of the growth phase. Combined with the modeled evolution of the magnetic configuration, this mechanism consistently explains the main features of the electron precipitation. ¿ American Geophysical Union 1990