Current-driven electromagnetic ion cyclotron instability at substorm onset

ULF waves at frequencies of the order of the proton gyrofrequency are systematically detected at the early development of substorm breakups. The observed characteristics of these ULF waves, namely their polarization and δE/δB ratio are consistent with being electromagnetic waves driven unstable by a parallel current. In order to take into account properly wave particle interactions, a kinetic approach is used. We show that a parallel drift between electrons and ions leads to a strong instability, resulting from a coupling between the shear Alfvén (SA) mode and the fast magnetosonic mode via this drift. We call it current-driven Alfvén 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 geostationary orbit before and during breakup. We conclude that even a modest parallel drift between electrons and ions (V_d), caused by a parallel current, can destabilize CDA waves. When the ratio between V_d/V_A (V_A being the Alfvén velocity) increases, the CDA mode couples with SA mode. These two modes have a substantial parallel electric field that leads to a fast parallel diffusion of the electrons. We suggest that this parallel diffusion leads to an interruption of the parallel current.