Spacecraft observations of magnetohydrodynamic turbulence in corotating interaction regions
Gulamali, M. Y. (2002) Spacecraft observations of magnetohydrodynamic turbulence in corotating interaction regions, Ph.D. thesis, Imperial College of Science, Technology and Medicine, University of London, London, U.K.
Abstract
During its unique voyage around the poles of the Sun, the Ulysses spacecraft observed two sets of well-developed corotating interaction regions (CIRs) at mid-heliographic latitudes. Because CIRs involve the interaction of plasmas from different source regions on the Sun they provide a novel and complicated environment in which to study the character and evolution of solar wind turbulence. Consequently, in this thesis we present a study of the nature of magnetohydrodynamic (MHD) turbulence within the solar wind streams associated with the CIRs observed by Ulysses.
We use Fourier spectral analysis and a wavelet based technique in order to describe MHD fluctuations within CIRs. We use this to carry out a comprehensive survey of the character of turbulence in the CIRs recorded by Ulysses. We also perform one-dimensional numerical simulations of the interaction of Alfven waves and MHD shocks in order to examine the nature of turbulent fluctuations within the compression regions of CIRs.
Our findings suggest that each CIR observed by Ulysses has a very complicated turbulent signature due to its specific structure, and the presence of transient events in the CIR. However, statistical analyses of the data show that the turbulent fluctuations in both sets of CIRs, were essentially the same, with small differences being attributable to the relative difference in the phase of the solar activity cycle when the measurements were made.
The results of our numerical studies suggest that the turbulent fluctuations in the compression regions of CIRs are dependent upon the nature of the shocks bounding the region, as well as turbulent fluctuations outside of the compression region. We find that the reduction in the absolute value of the normalised cross helicity in this region can be attributed to secondary waves generated at the shock.