Evolution of Turbulence Anisotropy in the Outer Heliosphere and Transport of Pickup-ion-associated Energy in Turbulence Channel : Voyager 2 Observations

The solar wind plasma environment in the outer heliosphere is different from the inner heliosphere where has been widely studied. An important factor influencing the turbulence evolution in the outer heliosphere is the pickup ions, primarily originated from neutral atoms from the interstellar medium. Pickup ions are not readily assimilated by the background solar wind plasma and thus provide extra free energies which can drive ion-scale instabilities. The unstable growing waves will end up taking part in the turbulent energy transport. However, how these pickup-ion-associated energies involve in turbulent cascade and influence turbulence evolution have yet to be studied. In this work, we study the solar wind turbulence evolution from 1 au to 33 au based on Voyager 2 magnetic field measurements. We study 305 time intervals listed in Pine et al. (2020). In all these time intervals, no ion-scale bumps are present in the turbulent spectra. We find that: (1) The perpendicular and trace power spectra (and ) still follow a Kolmogorov-like spectrum until 33 au while the parallel power spectrum transits from -2 to -5/3 at heliocentric distance R~10 au; (2) At periods 10 s <τ< 500 s, quasi-parallel propagation dominates in 1 au<R<7 au, with quasi-perpendicular propagation gradually emerging at R>5au. For R > 7 au, oblique propagation becomes a primary component. (3) At larger periods of τ>100 s, increases with propagation angle in 1 au<R<5 au, and in contrast decreases with propagation angle at R>5 au due to the enhanced power level at propagation angles smaller than . We suggest that such enhancement may derive from the injection of the wave energy from the pickup ion source into the background tubulent cascade , and the injected wave energy is transferred across scales withou leaving bumps in or .