Ulysses observations of magnetohydrodynamic turbulence in corotating interaction regions
Gulamali, M. Y., and P. J. Cargill (2001), Ulysses observations of magnetohydrodynamic turbulence in corotating interaction regions, J. Geophys. Res., 106, 15687–15699, DOI: 10.1029/2000JA000390.
During its mission around the poles of the Sun, the Ulysses spacecraft encountered a number of well-developed corotating interaction regions (CIRs) at midlatitudes. In this paper we discuss the character of magnetohydrodynamic (MHD) turbulence within defined regions of these CIRs. We examine magnetic field power spectra at a fixed wave number, and the normalized cross helicity, in order to describe solar wind fluctuations. We find that each CIR has a complicated turbulent signature. Magnetic field power spectra show evidence for the turbulent mixing of plasma at large distances from the Sun, and there are possible signs of instabilities arising at stream interfaces. Statistical analyses of our data set show that there does not appear to be any clear dependence of turbulence in the CIRs upon heliographic latitude but that there is a clear trend in magnetic field power and Alfvénicity with CIR structure. We also find that the variation of the solar cycle during Ulysses observations of these CIRs has a clear effect upon the turbulent character of the solar wind and thus provides further evidence that the source of the majority of turbulent fluctuations in the solar wind is the Sun.