Magnetic Turbulence Spectra and Intermittency in the Heliosheath and in the Local Interstellar Medium

Abstract

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.

Dispersive Fast Magnetosonic Waves and Shock‐Driven Compressible Turbulence in the Inner Heliosheath

“…However, as it is demonstrated in Figure 7, the spectral slope of magnetosonic turbulence in frequency domain is not the same as the spectral slope in wave number domain due to the breaking down of Taylor's hypothesis in the inner heliosheath. This implies that the observed turbulent magnetic field spectrum is not a classical forward‐cascade Kolmogorov spectrum, contrary to Fraternale et al (2019). The Voyager 2 turbulent magnetic spectrum (Figure 8c) also shows a narrow peak close to the proton gyrofrequency and its harmonics, which confirms the presence of ion‐ion hybrid resonance.…”

“…However, these minor ion species were not included in the numerical three‐fluid simulation. The observed turbulent magnetic spectrum is known to be affected by noise at frequencies above the proton gyrofrequency (Fraternale et al, 2019). Therefore, we cannot make a reliable comparison between the simulated and observed turbulent magnetic spectra in the high‐frequency dissipation range.…”

Intermittency and q-Gaussian Distributions in the Magnetic Field of the Very Local Interstellar Medium (VLISM) Observed by Voyager 1 and Voyager 2

“…Kolmogorov turbulence (Kolmogorov 1941) was observed in B of the VLISM (Burlaga et al 2015(Burlaga et al , 2018, during two relatively undisturbed "quiet" 468 day intervals. A spectral analysis of B in the heliosheath and in the VLISM through 2016 by Fraternale et al (2019) confirmed the existence of Kolmogorov turbulence over a limited frequency range and found evidence of intermittency.…”

Magnetic Field and Plasma Density Observations of a Pressure Front by Voyager 1 during 2020 in the Very Local Interstellar Medium

“…Smaller scale waves and turbulence exist within these features. Mathematical techniques such as Hilbert transforms and wavelet transforms (Zhao et al 2020) and Fourier methods (Fraternale et al 2019) are very valuable tools for studying the VLISM over a wide range of scales. However, it is also essential to understand the basic structure and dynamics of the large-scale flows on scales of hours to a decade presented in this paper.…”