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Session 15 - Structure and Collapse of Molecular Clouds: Theory.
Display session, Monday, January 13
Much of the turbulence in the interstellar medium is magnetohydrodynamic in nature. In some cases this turbulence can be modeled as an ensemble of Alfven waves. One of the potentially important processes affecting such waves and turbulence is the parametric decay instability, in which an Alfven wave decays into another Alfven wave and an ion acoustic wave. This mechanism is important because the daughter ion acoustic wave will be heavily damped, so the instability provides a channel for damping otherwise dissipationless Alfven waves. We recently completed a search for evidence of this instability in the solar wind. The decay instability appeared to be weakly present in 2 of 20 data intervals examined. In several of the remaining 18 cases conditions seemed to be theoretically conducive for the instability, but it appeared not to have occurred. We discuss probable reasons for the weakness of the instability in the solar wind, and the implications for its operation in the interstellar medium. The most likely mechanisms of suppression are a broad bandwidth of the parent waves and kinetic effects due to non-Maxwellian particle distribution functions. We suggest that the decay instability is most likely to occur and be an important process in regions of low plasma \beta and spatially-distributed, monochromatic wave generation. Contracting star formation clouds might provide such a medium.
Program listing for Monday