HR: 08:35h
AN: NG31C-03    [Abstracts]
TI: Self-generation of phase coherence in multiply-coupled triplet system
AU: * Hada, T
EM: hada@esst.kyushu-u.ac.jp
AF: ESST, Kyushu University, 6-1 Kasuga Koen, Kasuga, 816-8580 Japan
AU: Nariyuki, Y
EM: nariyuki@esst.kyushu-u.ac.jp
AF: ESST, Kyushu University, 6-1 Kasuga Koen, Kasuga, 816-8580 Japan
AB: We have demonstrated in previous presentations that large amplitude MHD waves observed in the solar wind are not completely phase random (as assumed in quasi-linear theories), but are almost always phase correlated to a certain degree. This presumably is a consequence of nonlinear interaction among the MHD waves. Recently we have developed a method to quantitatively evaluate the phase coherence among waves from a given turbulence time series, via comparison of structure functions of the original data as well as its phase shuffled and phase coherent surrogates (Hada et al., 2003; Koga and Hada, 2003). From a statistical study using Geotail magnetic field data, it is concluded that the phase correlation is almost always non-zero, is enhanced when the turbulence energy is high (nonlinear interaction is strong), and that the waves only within a finite frequency range are responsible for the generation of the phase coherence. In order to understand the physical mechanism of the phase coherence generation, we have constructed a simple model of weak turbulence by coupling multiple triplets, which are the minimum units of nonlinear interaction among the modes (nonlinear interaction of three waves in resonance). According to our numerical simulations, long time statistical behavior of the coupled system critically depends on how the triplets are connected: if the connection is given in such a 'physical' way that the frequency of each mode can be uniquely determined, the system approaches the SOC state as time elapses; otherwise, the system does not evolve to any statistically interesting state. Furthermore, adjacent modes often tend to phase synchronize, as the modes exchange energy (quanta) between them. Implications of the results to the phase coherence detected in the solar wind data will be discussed.
DE: 7839 Nonlinear phenomena
DE: 7863 Turbulence
DE: 2149 MHD waves and turbulence
SC: Nonlinear Geophysics [NG]
MN: 2004 AGU Fall Meeting