Power partition between dispersive fast-mode branches in heliosheath turbulence

Determine how turbulent fluctuation power is partitioned between the pickup-ion-driven high-frequency fast magnetosonic branch (HFF) and the solar-wind-ion-driven low-frequency fast magnetosonic branch (LFF) in the inner heliosheath, using the three-fluid warm plasma dispersion relation for perpendicular propagation, including the dependence of this partition on wavenumber, to enable quantitative construction of the 4D frequency–wavenumber spectrum and consistent comparison with Voyager observable frequency spectra.

Background

The paper models the spacecraft-observable turbulence spectrum in the inner heliosheath using a full 4D frequency–wavenumber framework that incorporates both isotropic MHD Alfvén and fast modes and, critically, two dispersive fast magnetosonic branches derived from a three-fluid description with pickup ions, solar wind ions, and electrons. These two branches— a high-frequency fast mode associated with pickup ions (HFF) and a low-frequency fast mode associated with solar wind ions (LFF)—are present in the perpendicular propagation limit and have distinct dispersion characteristics.

Because the observable spectrum depends sensitively on how fluctuation power is distributed between HFF and LFF, the authors adopt a simplifying assumption of equal power for the two branches to proceed with spectral modeling. They explicitly note, however, that the true power partitioning is not known, and resolving it is essential for accurately reconstructing the 4D spectrum and interpreting features such as the spectral bump near the proton gyrofrequency in Voyager data.