Relation between Field Aligned Currents, Electrons and ULF Waves

ULF waves at frequencies of the order of the proton gyrofrequency have been identified as Current Driven Alfvén Waves (CDAW) resulting from a coupling between the Shear Alfvén (SA) and Fast MagnetoSonic modes via a parallel drift of the electrons with respect to the ions. These waves are systematically observed just at substorm onset at the geostationary orbit, as well as further in the Earth magnetic tail, and in the auroral region. Magnetic measurements (DC and waves) at four separate CLUSTER locations allow us to identify the free energy source of these waves by measuring the current density near CLUSTER apogee, and to study their propagation along the field lines. When the waves propagate along the field lines, they undergo a conversion mode into SA waves. CDA and SA waves have a parallel electric field component which is > 2 % of the perpendicular component. This parallel electric field component leads (via Landau damping), to strong parallel diffusion of the electrons along closed field lines, as evidenced by the electron distribution. The diffusion time is much shorter than the bounce period of electrons, thus it is a very efficient process to reduce the parallel current.