Identification of nonlinear processes in space plasma turbulence.

Frequency domain analysis tools have been developed to analyse simultaneous multi-point measurements of developed space plasma turbulence. The Coherence Length technique enables the scale length for plasma wave structures to be measured from magnetic field measurements. The coherence length defines a length scale for the measurement of wave phenomena. Single satellite measurements can be used, the technique becoming more reliable with higher numbers of satellites. The technique is used to identify coherence lengths for waves observed in the magnetic field near the bow shock by the dual AMPTE-UKSIAMPTE-IRM satellites, and for mirror wave structures observed in the magnetic field in the magnetosheath by the dual ISEE-lIISEE-2 satellites. The Transfer Function Estimation technique enables the transfer of energy between plasma waves to be measured, from simultaneous dual-point measurements, resulting in linear growth / damping rates and second-order wave coupling. The technique is improved by replacing the Least Squares method for inversion with Regularisation. The technique is applied to simultaneous magnetic field measurements near the bow shock by the AMPTE-UKSIAMPTE-IRM satellites, where a linear instability in the wave field is identified, which is attributed to an ion anisotropy instability, and accompanying sequence of second-order three-wave coupling processes is also identified, which dissipates the energy from the linear instability. The Wave vector Determination technique enables the identification of wave vectors from simultaneous four-point measurements. The availability of four-point measurements means that the reliance on Minimum Variance Analysis, and that of only being able to use magnetic field measurements, is removed, the wave vector can be determined unambiguously directly from the magnetic field measurements. The technique can identify between waves of different frequency, and waves at the same frequency but propagating in different directions. The technique is applied to simultaneous observations of the electric field by the four-point ii Cluster II satellites, enabling the determination of the wave vector and the identification of a mirror mode structure, solely from the electric field measurements. Chapter 1 introduces the solar-terrestrial environment, briefly describing exploration of this environment by man-made satellites and listing some aims of the analysis of data collected by the satellites. Chapter 2 elaborates on what is meant by data analysis; Spectral Transforms are introduced and described, with a comparison made between Fourier Transforms and Wavelet Transforms, before a review is made of current data analysis techniques for satellite data. Chapter 3 defines and focuses attention on the objectives of this thesis, which are addressed in the following three chapters. Chapter 4 investigates the coherence length of plasma waves through use of the Wavelet Transform and the Fourier Shift Theorem. Chapter 5 makes estimates of wave Transfer Functions, replacing an established Least Squares inversion technique with a Regularisation inversion. Chapter 6 uses a method to determine wave propagation directions, from multi-satellite data, that has not been applied before due to the lack of availability of suitable data sets. Chapter 7 summarises the preceding chapters. The Appendices contain reprints of papers resulting from, and relating to, this research.