  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
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 Alfven and fast mode
turbulence are considered to represent the heliosheath fluctuations. We also
include two dispersive fast wave modes derived from a three-fluid theory. We
find that (1) magnetic fluctuations in the inner heliosheath are less
compressible than previously thought. An isotropic turbulence spectral model
with about 1/4 in compressible fluctuation power is consistent with the
observed magnetic compressibility in the heliosheath; (2) the hot pickup ion
component and the relatively cold solar wind ions induce two dispersive fast
magnetosonic wave branches in the perpendicular propagation limit. Pickup ion
fast wave may account for the spectral bump near the proton gyrofrequency in
the observable spectrum; (3) it is possible that the turbulence wavenumber
spectrum is not Kolmogorov-like although the observed frequency spectrum has a
-5/3 power law index, depending on the partitioning of power among the various
wave modes, and this partitioning may change with wavenumber.
