HR: 1340h
AN: SM43C-1238    [Abstracts]
TI: A Study of Energy and Momentum Transports in the Event of MHD Kelvin-Helmholtz Instability
AU: Lai, S
EM: lucky@jupiter.ss.ncu.edu.tw
AF: Institute of Space Science, National Central University, Chung-li, 320-01 Taiwan
AU: * Lyu, L
EM: lyu@jupiter.ss.ncu.edu.tw
AF: Institute of Space Science, National Central University, Chung-li, 320-01 Taiwan
AB: It is commonly believed that the surface wave developed from the Kelvin-Helmholtz (K-H) instability at the magnetopause plays an important role in the momentum and energy transports during the period of northward IMF (interplanetary magnetic field). Previous simulation studies of the K-H instabilities confirmed this hypothesis. But the momentum transport, deduced from previous simulation results [Miura, 1990; 1992], decreases with increasing sonic Mach number, which is in conflict with the theoretical prediction proposed by Pu and Kivelson [1983]. In this study, energy and momentum transports in the events of MHD K-H instabilities are studied by means of two-dimensional MHD simulation. An MHD tangential discontinuity (TD) is placed at the center of our simulation domain. The TD is of finite thickness with both magnetic shear and velocity shear across the TD. Our simulation results indicate that when the fast-mode Mach numbers on both sides of the TD are less than 1, the K-H instability can grow into vortices or kink-type surface waves. Energy and momentum transports across the TD can be found during the growth phase of these low-Mach-number K-H instabilities. After the formation of vortices in the transition layer, the energy flux also show vortex structures. As a result, the net average energy flux diminishes during the saturation phase of low-Mach-number K-H instability. When the fast-mode Mach numbers on either side of the TD is greater than 1, nonlinear fast-mode plane waves are developed from the ridges of the surface waves and extended distance from the TD. The distributions of net average energy flux and momentum flux indicate that the generation and propagation of the nonlinear fast-mode plane waves provide a steady and efficient mechanism in transferring momentum and energy across a TD during the growth phase and saturation phase of the high-Mach-number K-H instability. When the background magnetic fields are nearly perpendicular to our simulation plane, the wave mode conversion between the fast mode and the Alfven mode are suppressed. In these cases, the energy fluxes obtained in our simulations are nearly parallel to the group velocity of the nonlinear fast-mode wave, which are in good agreement with previous theoretical prediction proposed by Pu and Kivelson [1983].
DE: 2724 Magnetopause and boundary layers
DE: 7811 Discontinuities (2109)
DE: 7836 MHD waves and instabilities (2149, 2752, 6050)
DE: 7839 Nonlinear phenomena (4400, 6944)
DE: 7859 Transport processes
SC: SPA-Magnetospheric Physics [SM]
MN: Fall Meeting 2005