Current-driven electromagnetic ion cyclotron instability at substorm onset

ULF waves at frequencies of the order of the proton gyrofrequency are syste matically detected at the early development of substorm breakups. The obser ved characteristics of these ULF waves, namely their polarization and delta E/delta B ratio are consistent with being electromagnetic waves driven uns table by a parallel current. In order to take into account properly wave pa rticle interactions, a kinetic approach is used. We show that a parallel dr ift between electrons and ions leads to a strong instability, resulting fro m a coupling between the shear Alfven (SA) mode and the fast magnetosonic m ode via this drift. We call it current-driven Alfven instability (CDA). We have carried out a parametric study of this current-driven electromagnetic instability in a parameter range adapted to conditions prevailing at the ge ostationary orbit before and during breakup. We conclude that even a modest parallel drift between electrons and ions (V-d), caused by a parallel curr ent, can destabilize CDA waves. When the ratio between V-d/V-A (V-A being t he Alfven velocity) increases, the CDA mode couples with SA mode. These two modes have a substantial parallel electric field that leads to a fast para llel diffusion of the electrons. We suggest that this parallel diffusion le ads to an interruption of the parallel current.