CHARACTERIZATION OF LOW-FREQUENCY OSCILLATIONS AT SUBSTORM BREAKUP

Field and particle data recorded on the geostationary satellite GEOS 2 are used to investigate the electric and magnetic signatures of a sub storm characterized by a dispersionless injection of energetic electro ns and ions. Three types of field variations are observed: (1) Long-pe riod oscillations with period of similar to 300 s, interpreted as osci llations of entire field lines. These oscillations develop as second h armonic standing waves and correspond to coupled shear Alfven-slow mag netosonic modes. They grow after the most active period of the breakup . (2) Short-period transient Oscillations with periods of similar to 4 5 - 65 s, interpreted as wave modes trapped in a current layer which d evelops prior to the substorm breakup and is disrupted at breakup. The se oscillations also correspond to a coupled shear Alfven-slow magneto sonic mode (coupled via magnetic held curvature effects in a high-beta plasma). The short-period transient oscillations are only observed du ring the most active period of the breakup. (3) A nonoscillatory sharp increase observed on both the parallel magnetic component and the ene rgetic ion flux, averaged over one satellite rotation, interpreted as evidence for the fast magnetosonic mode which in view of the simultane ous large impulsive increase in the azimuthal electric field, appears to propagate radially outwards, transporting the substorm breakup down tail.