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Title: Multi-spacecraft measurement of turbulence in the solar wind
Author: Osman, Kareem
ISNI:       0000 0001 3460 6981
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2008
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A novel multi-spacecraft technique, applied to measurements made by the four Cluster spacecraft in the solar wind, is used to determine the field-aligned anisotropy of magnetohydrodynamic inertial range turbulence. Time-lagged two-point correlation functions are used to construct spatial auto-correlation functions. These are used in conjunction with a phenomenological elliptical scaling model to estimate the ratio of field-parallel to total correlation lengths, obtaining a mean value of 0.61 ± 0.02. A simulated turbulent field model is also fitted to the spatial auto-correlation functions in order to estimate the power in the 2D fluctuations, obtaining a mean value of (79 ± 3)%. The results obtained from both models, assuming a slab-2D paradigm, are consistent with solar wind fluctuations being anisotropic with energy mainly in wave vectors perpendicular to the mean magnetic field. Eight intervals of multi-point magnetic field data are analysed, and the degree of variation in the estimates of anisotropy about the mean is larger than expected for both models. This variation is not correlated with the solar wind velocity or the plasma beta. However, these anisotropy estimates are correlated between different field components, suggesting that the assumption of axisymmetry is valid. An alternative multi-spacecraft approach, where time-lagged second order structure functions are used to estimate the field-aligned anisotropy of the power levels and the spectral index, is also presented. The mean value obtained for the power in the 2D fluctuations, (93 ± 15)%, is consistent with the simulated turbulent field results, while the observed anisotropy in the spectral index suggests that multiple energy transfer mechanisms are present in solar wind turbulence. Results from all three analysis techniques are discussed and found to be correlated with each other. This suggests that the observed variation in the anisotropy estimates could be a physical effect that different data intervals exhibit different degrees of anisotropy.
Supervisor: Cargill, Peter Sponsor: Science and Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral