Large-Amplitude Alfven Waves Observed in the Magnetotail and Associated Magnetospheric Signature

Alfven waves in the magnetotail are considered to be associated with the auroral activity at the ionospheric altitudes. We have investigated the electric and magnetic field variations measured by Geotail in the mid-tail region, 10-30 Re, for 4 years, from 1997 to 2000. We found large-amplitude electric field fluctuations, which have peaks over 10 mV/m, and the corresponding magnetic field fluctuations having the components perpendicular to the ambient magnetic field. We made linear fitting analysis between the electric and magnetic fields, and identified these fluctuations as Alfven waves from the comparison of the local Alfven speed with the ratio between the electric and magnetic field amplitudes. The large-amplitude Alfven waves propagate in the direction coinciding with the plasma flow, which is often earthward in the distances of 10-25 Re, and tailward at > 25 Re. These Alfven waves are found to be accompanied by either of following four signatures of plasma and magnetic field; the plasma-sheet boundary crossing, the earthward ion beam, the tailward ion beam and the dipolarization of the magnetic field. The Alfven waves with large Poynting energy are often accompanied by the ion beams. It is suggested that there are two mechanisms for the generation of the large-amplitude Alfven waves. One is the fluctuations of field and plasma motion occurred by the dipolarization at locations close to the central plasma sheet; second is the ion-cyclotron instability by the ion beams. In the latter case, the Alfven waves seem to diminish on the way of the propagation by providing their energy to thermal plasma component.