Physics – Geophysics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsm11c1175d&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #SM11C-1175
Physics
Geophysics
7839 Nonlinear Phenomena, 7843 Numerical Simulation Studies, 7863 Turbulence, 7867 Wave/Particle Interactions, 7871 Waves And Instabilities
Scientific paper
Despite an abundance of data on plasma turbulence, obtained either by direct space observations or from computer simulation, most of the data are interpreted only in a qualitative way, rather than by a detailed analysis that would allow a quantitative comparison with theory. For such a comparison one needs to obtain, as a key ingredient, reliable wave spectra as a function of wave number and frequency. The free energy sources, linear instability mechanisms and nonlinear coupling mechanisms that generate these wave spectra should also be identified. In the case of micro turbulence these mechanisms depend on details of the particle distribution functions. Even the nature of wave modes, not only wave growth rates, may change as the plasma evolves. A particle simulation in which an electron beam excites a variety of wave modes is used as an example for such an analysis. The model corresponds to proposed mechanisms for ion conic generation on auroral field lines. The rather rapid evolution of plasma and turbulence requires that the spectral analysis is carried out over time intervals that are sufficiently short compared to time scales for spectral changes, whereas for statistical reasons and good frequency resolution long sampling intervals would be desirable. Straightforward periodograms are unsatisfactory under these conditions, even when applying windows (tapers) to the wave trains, in order to reduce spectral leakage. Modern spectral analysis methods which were mostly developed in the geophysics context, such as the maximum entropy method and the multiple taper method, can yield far better results. They are adopted for the analysis of plasma turbulence, in particular in connection with particle simulation codes, although, with other data limitations, the considerations mostly apply also to observations. Particular attention is paid to statistical tests for spectral lines which may correspond to eigenmodes (instabilities) of the plasma. For reliable results it is usually advisable to try several ways of spectral analysis. This requires post processing in the case of simulation and special methods to keep the data output manageable, which we discuss.
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