Authors: Lingling Zhao (University of Alabama in Huntsville), Gary Zank (University of Alabama in Huntsville), Merav Opher (Boston University), Berci Zieger (Boston University), Hui Li (Los Alamos National Laboratory), Vladimir Florinski (University of Alabama in Huntsville), Laxman Adhikari (University of Alabama in Huntsville), Xingyu Zhu (University of Alabama in Huntsville), and Masaru Nakanotani (University of Alabama in Huntsville)

Magnetic field fluctuations measured in the heliosheath by the Voyager spacecraft are often characterized as compressible. as indicated by a strong fluctuating component parallel to the mean magnetic field. However, the interpretation of the turbulence data faces the caveat that the standard Taylor’s hypothesis is invalid because the solar wind flow velocity in the heliosheath becomes subsonic and slower than the fast magnetosonic speed, given the contributions from hot pickup ions in the heliosheath. We attempt to overcome this caveat by introducing a 4D frequency-wavenumber spectral modeling of turbulence, which is essentially a decomposition of different wave modes following their respective dispersion relations. Isotropic Alfvén and fast mode turbulence are considered to represent the heliosheath fluctuations. In addition, we also include two dispersive fast wave modes derived from the three-fluid theory. This allows us to make several revelations. First, magnetic fluctuations in the inner heliosheath are less compressible than previously thought. An isotropic turbulence spectral model with about 25% in compressible fluctuation power is consistent with the observed magnetic compressibility in the heliosheath. Second, the hot pickup ion component and the relatively cold solar wind ions may induce two dispersive fast wave branches. They may account for the “observed” spectral bump near the proton gyrofrequency, and which branch plays a role depends on the division in power between the two. Third, it is possible that the turbulence wavenumber spectrum is not Kolmogorov-like even though the observed frequency spectrum has a -5/3 power-law index. Future turbulence modeling efforts in the heliosheath are needed to jointly characterize the power partitioning among different kinds of wave modes.