Characterization of small scale turbulence observed at substorm onsets: Relationships with parallel acceleration of particles

Abstract During the onset of magnetospheric substorms in the night sector, impulsive fluctuations are observed in the ULF range at geostationary orbit. A detailed analysis of the electric and magnetic components, simultaneously detected on board the ESA GEOS-2 spacecraft, permits identification of three different waves: (i) Quasi Electrostatic waves (QES) and (ii) magnetosonic waves, both of which exhibit a predominant component parallel to the DC magnetic field at frequencies above the proton gyrofrequency f H + and (iii) transverse shear Alfven waves propagating azimuthally at frequencies below f H + . Comparison with particle measurements (ions and electrons) in different energy ranges shows that QES waves are related to electron precipitation leading to auroras at the footprint of the GEOS field line, whereas the magnetosonic waves are observed in the ring current where energetic ion and electron fluxes are large.