NEUTRAL SHEET OSCILLATIONS AT SUBSTORM ONSET

Using magnetometer and plasma instrument data of the Active Magnetospheric Particle Tracer Explorers/Ion Release Module satellite obtained in the plasma sheet, we investigate three events characterized by compressional narrowband fluctuations of the magnetic field with spectral peaks at periods of 1-2 min. Since such events are only observed in the vicinity of the neutral sheet, ave call them neutral sheet oscillations. The oscillation starts some minutes before or after the onset of a magnetospheric substorm. Exceeding 10 nT, the fluctuation of the magnetic field is comparable to the background field. The compressional component of the magnetic fluctuation is distinctly greater than the two transverse components. The proton and electron pressure exhibit fluctuations that are in antiphase with the fluctuation of the magnetic pressure. In all three events the oscillation is accompanied by an earthward directed high-speed flow with a typical velocity of 400-800 km/s and a typical duration of 2 min. In the course of the events the elevation of the magnetic field increases by more than 40 degrees. We interpret the neutral sheet oscillations as the transient response of the plasma sheet to the formation of the substorm current wedge at substorm onset. The formation of the current wedge causes a damped oscillation of the dawn-to-dusk electric field with a period of 1-2 min which is associated with plasma motions at the same period. In the vicinity of the neutral sheet, where the plasma has strong vertical gradients, those motions lead to fluctuations of the magnetic field and of the ion and electron pressure seen by the spacecraft.