We study the development of substorm breakups characterized by dispersionless injections of energetic particles at the geostationary orbit. The corresponding magnetic signature is a fast change from tail-like to dipole-like configuration with transient superimposed low frequency oscillations (T~1 mn). We show that intense waves (ΔB≈1 nT) with shorter periods (1 s) systematically develop at breakup, and that their intensification is strongly related to the depolarization and to the fast increase of energetic electrons. These higher frequency (F~1 Hz) waves appear as short lasting bursts, strongly confined across the magnetic field. Hence they look like kinetic Alfv¿n waves and are likely to have finite parallel electric fields, thereby resonating with electrons. We compute the diffusion coefficient and show that electrons are heated along the parallel direction and can gain up to 5 keV in a few tens of seconds. This fast parallel diffusion of electrons leads to cancellation of the parallel current and therefore to a complete modification of the current system. ¿ 2000 American Geophysical Union