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We examine Advanced Composition Explorer and Helios 1 data in search of evidence for an anisotropic spectrum of interplanetary magnetic and velocity field fluctuations. Specifically, we focus on the power-law indices of the fluctuation spectra and associated second-order structure functions and ask whether the index varies systematically with the angle between the mean magnetic field and the wind velocity. We extend previous results to show convincingly that it does not. Several popular theories for magnetohydrodynamic turbulence predict a significant variation as part of the turbulent cascade dynamic. We offer some observations on why the predicted anisotropy is not present.
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Tessein et al. (2009) studied this question.
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