The understanding of inertial-scale dynamics in the heliosheath is not yet thorough. Magnetic field fluctuations across the inner heliosheath and the local interstellar medium are here considered to provide accurate and highly resolved statistics over different plasma conditions between 88 and 136 AU. By using the unique in situ 48-s measurements from the Voyager Interstellar Mission, we investigate different fluctuation regimes at the magnetohydrodynamic (MHD) scales, down to the MHD-to-kinetic transition. We focus on a range of scales exceeding five frequency decades (5 × 10 −8 < f < 10 −2 Hz), which is unprecedented in literature analysis. A set of magnetic field data for eight intervals in the inner heliosheath, in both unipolar and sector regions, and four intervals in the local interstellar medium is being used for the analysis. Results are set forth in terms of the power spectral density, spectral compressibility, structure functions and intermittency of magnetic field increments. In the heliosheath, we identify the energy-injection regime displaying a ∼ 1/f energy decay, and the inertial-cascade regime. Here, the power spectrum is anisotropic and dominated by compressive modes, with intermittency that can reach kurtosis values up to ten. In the interstellar medium the structure of turbulence is anisotropic as well, with transverse fluctuations clearly prevailing after May 2015. Here, we show that intermittent features occur only at scales smaller than 10 −6 Hz.

The interaction of two isotropic turbulent fields of equal integral scale but different kinetic energy generates the simplest kind of inhomogeneous turbulent field. In this paper we present a numerical experiment where two time decaying isotropic fields of kinetic energies E1 and E2 initially match over a narrow region. Within this region the kinetic energy varies as a hyperbolic tangent. The following temporal evolution produces a shearless mixing. The anisotropy and intermittency of velocity and velocity derivative statistics is observed. In particular the asymptotic behavior in time and as a function of the energy ratio E_{1}E_{2}-->infinity is discussed. This limit corresponds to the maximum observable turbulent energy gradient for a given E1 and is obtained through the limit E_{2}-->0 . A field with E_{1}E_{2}-->infinity represents a mixing which could be observed near a surface subject to a very small velocity gradient separating two turbulent fields, one of which is nearly quiescent. In this condition the turbulent penetration is maximum and reaches a value equal to 1.2 times the nominal mixing layer width. The experiment shows that the presence of a turbulent energy gradient is sufficient for the appearance of intermittency and that during the mixing process the pressure transport is not negligible with respect to the turbulent velocity transport. These findings may open the way to the hypothesis that the presence of a gradient of turbulent energy is the minimal requirement for Gaussian departure in turbulence.

A numerical experiment on the interaction between different decaying homogeneous and isotropic turbulence is described. In the absence of kinetic energy production, the intermediate asymptotics of the turbulent shear-free mixing layer can be observed. The first aim of the experiment is to verify the existence of the intermittency or of the Gaussian asymptotic state in the case of the absence, or weak presence, of a lengthscale gradient. The second aim is to analyse the effects that are due to the difference between the spectral distribution of the interacting turbulence fields, which introduces the presence of the gradient of integral scale into the initial condition.It can be observed that the homogeneity of the integral length across the shearless layer is not a sufficient condition to obtain the Gaussian asymptotic state. In fact, if the macroscale gradient is suppressed by considering turbulence with similar spectra, it is apparent that the intermittency increases with the energy gradient. Furthermore, by independently varying the initial energy level and distribution over the wavenumbers, two turbulence fields can be joined with an initial difference of integral scale either opposite to or concordant with the gradient of the turbulent kinetic energy. It is found that the intermittency and the depth of penetration by the eddies from the high-energy region increase when the energy and lengthscale gradients are concordant and decrease when they are opposite. Therefore, the most efficient process of mixing takes place when the spectra of two mixed fields differ in the lowest wavenumbers.

The generation of small-scale anisotropy in turbulent shearless mixing is numerically investigated. Data from direct numerical simulations at Taylor Reynolds' numbers between 45 and 150 show not only that there is a significant departure of the longitudinal velocity derivative moments from the values found in homogeneous and isotropic turbulence but that the variation of skewness has an opposite sign for the components across the mixing layer and parallel to it. The anisotropy induced by the presence of a kinetic energy gradient has a very different pattern from the one generated by an homogeneous shear. The transversal derivative moments in the mixing are in fact found to be very small, which highlights that smallness of the transversal moments is not a sufficient condition for isotropy.

A new matched asymptotic expansion for the intermediate and far flow behind a finite body / TORDELLA D.; M. BELAN.-In: PHYSICS OF FLUIDS.