[en] The authors present space and ground based observations made during the growth phase and the onset of a substorm on August 31, 1986. Roughly 20 minutes after the var-epsilon parameter at the magnetopause had exceeded 10¹¹ W, the AMPTE Charge Composition Explorer spacecraft observed an increase in energetic particle fluxes consistent with magnetic field depolarization. The craft was close to magnetic midnight at a geocentric distance of 8.7R_E. The event had the initial signature of a substorm onset, but it did not lead to a full-scale substorm expansion based on several ground based observations. There were no large particle injection events at geostationary orbit. After another 20 minutes the event did enter a normal substorm expansion phase. The authors interpret the initial activation as a open-quotes pseudobreakupclose quotes. They correlate observations made by spacecraft in the near-Earth plasma sheet, with ground based observations of the ionospheric development from magnetometer and electric field measurements from the STARE radar. The strength and the consequences are concluded to be the main differences of pseudobreakups and ordinary breakups