Wave dynamics in the geomagentic tail.

Abstract

The Geomagnetic tail is the region of the earth's magnetosphere stretched by the solar wind away from the Sun. The stretched Geomagnetic tail acts as a huge magnetic energy reservoir powering a variety of processes, for instance the substorm and the aurora, which affect the entire magnetosphere. This thesis presents analyses of the wave dynamics of the geomagnetic tail at the spatial scale from the Magnetohydrodynamics (MHD) to the ion gyroradius and the temporal scale from the ion gyro-period to many gyro-periods. The results provide new insights into the energy flow process in the magnetotail reconnection. In addition, the implication of these results sheds light on the energy transport from the geomagnetic tail to the aurora zone. The bidirectional outflowing ion jet is a diagnostic signal of magnetic reconnection. We present a Cluster spacecraft study of the intense surface waves in the earthward and tailward reconnection outflow jets in the geomagnetic tail. The four Cluster spacecraft are used to determine quantitatively the scale size and phase velocity of waves with spacecraft frequencies from 3 times 10^{-2} Hz to 1 Hz and spatial scales ranging from much larger (x50) than to comparable to the H+ gyroradius scale. The wave phase velocity relative to the spacecraft frame is directed mainly in the equatorial plane and it tracks the variation in the direction of the jet's velocity projection perpendicular to the magnetic field lying in the xy-gse plane. The surface waves are phase standing in the flow normal to the plasma sheet boundary, but partially or entirely convected by the flow in the plane of the plasma sheet (xy-gse). The surface wave is consistent with a Kelvin Helmholtz instability driven by the gradient in the normal direction of the component of the reconnection ion jet velocity perpendicular to magnetic field. E/B ratios provide evidence that dispersive Alfven waves are excited at small scales. Analysis of electric and magnetic field data shows that the wave perturbations are associated with strong Alfvenic Poynting flux radiated away from the reconnection region toward Earth along the geomagnetic field. The mapped values (to 100 km altitude) of Poynting flux ( 100 ergs/cm^2s) and longitudinal scales (10-100 km) of the waves suggest that the observed waves and their motions are an important boundary condition in determining both the energetics of the aurora and their complex motions in the night sky. The Harris current sheet is a good approximation of the Geomagnetic Tail configuration. We present a theoretical analysis of the linear Alfven eigenmode dynamics of a Harris current sheet. The implication of this theory in the context of magnetic reconnection is not presented. Alfven eigenmodes are confined by the Harris current sheet in the same way that quantum mechanical waves are confined by the tanh^2 potential. Although the Alfven eigenmodes are confined in the current layer, their dynamics is interrelated with the global-scale information of the current sheet. The linear dynamics of the Harris current sheet is described as a eigenmode-source coupling process, during which magnetic energy can be converted into plasma energy and the first-order magnetic configuration of the Harris sheet alters.