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How to Define the Mean Square Amplitude of Solar Wind Fluctuations With Respect to the Local Mean Magnetic Field
Author(s) -
Podesta John J.
Publication year - 2017
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2017ja023864
Subject(s) - turbulence , amplitude , solar wind , physics , mean field theory , magnetic field , magnetohydrodynamics , field (mathematics) , computational physics , statistical physics , dipole model of the earth's magnetic field , meteorology , classical mechanics , mathematics , interplanetary magnetic field , optics , quantum mechanics , pure mathematics
Over the last decade it has become popular to analyze turbulent solar wind fluctuations with respect to a coordinate system aligned with the local mean magnetic field. This useful analysis technique has provided new information and new insights about the nature of solar wind fluctuations and provided some support for phenomenological theories of MHD turbulence based on the ideas of Goldreich and Sridhar. At the same time it has drawn criticism suggesting that the use of a scale‐dependent local mean field is somehow inconsistent or irreconcilable with traditional analysis techniques based on second‐order structure functions and power spectra that, for stationary time series, are defined with respect to the constant (scale‐independent) ensemble average magnetic field. Here it is shown that for fluctuations with power law spectra, such as those observed in solar wind turbulence, it is possible to define the local mean magnetic field in a special way such that the total mean square amplitude (trace amplitude) of turbulent fluctuations is approximately the same, scale by scale, as that obtained using traditional second‐order structure functions or power spectra. This fact should dispel criticism concerning the physical validity or practical usefulness of the local mean magnetic field in these applications.