Measurements of energetic particles and the geomagnetic field by the geostationary satellite GEOS 2 were used to investigate the plasma and magnetic field parameters at substorm onset. The distribution of energetic ions and electrons are found to be statistically isotropic at the nightside synchronous orbit. The number density and pressure of electrons Ne and Pe are much lower than Ni and Pi of ions. Electrons and ions increase substantially at substorm onset when compared with magnetically quiet days. Earthward pressure gradients of energetic ions with scale length of ~10 ri (ri=ion gyroradius) are commonly observed during the substorm growth phase. The magnetic field lines are stretched out tailward with a curvature radius in the equatorial plane being considerably smaller than that in the dipole field. Plasma &bgr; values increase to the order of 1. All these parameters are found to be consistent with the development of the ballooning instability in connection with substorm onsets. ¿ American Geophysical Union 1992