A detailed case study was carried out of a magnetospheric substorm on June 23, 1979. Satellite, balloon and ground based particle and field data were used to correlate observations covering an extended area both in longitude and in latitude. The loading phase of the substorm was characterized by an equatorward convex, southward moving precipitation boundary. During the equatorward motion a hardening of the electron spectrum is seen. We show that betatron acceleration is stronger than Fermi acceleration for earthward convecting particles in a dipolelike field, and that the reverse is true for a taillike field. It is argued that the observed precipitation front represents an ionospheric mapping of an injection boundary. Energy dependent increases of the ion phase space density during the loading phase can be explained in terms of an earthward displacement of the inner edge of the neutral sheet current. Pi 2 pulsation on the satellite and on the ground are present both at substorm onset and at subsequent intensifications. Evidence is given for plausible correlation between changes in the IMF and variations in ionospheric electric fields as observed on balloons. It is suggested that changes of the IMF and variations in ionospheric electric fields as observed on balloons. It is suggested that changes of the IMF from south to north may have triggered the substorm and expansion phase intensifications. ¿ American Geophysical Union 1987