Expansion Effects in Voyager Magnetic Field Spectra

Analysis of Voyager 1 & 2 magnetic field data from 1 to 15 AU reveals an unexpected anisotropy within the inertial range of the power spectrum. Unlike the results from 1 AU presented by Belcher & Davis [JGR, 76, 3534, 1971], which revealed a variance anisotropy that is essentially isotropic in the 2D plane perpendicular to the mean magnetic field and which was further addressed by Bieber et al. [JGR, 101, 2511, 1996] as arising from an admixture of 1D and 2D wave vector anisotropies, we find a highly anisotropic variance anisotropy in the perpendicular plane. We account for this by considering expansion effects which reduce the fluctuation energy for the magnetic component in the Radial direction. We explore the possibility that expansion at heliocentric distances less than 15 AU results in this new form of variance anisotropy and we raise the suggestion that the same effect may be contributing to the earlier observations of Belcher & Davis.