Shock-Mediated Turbulent Cascade and Magnetic Field Structures Observed by Voyagers in the Heliospheric Boundary Layer

This study presents the first Voyager 1 observation of magnetic turbulence intermittency and fine-scale structures associated with the propagation of a shock wave in the outer heliosheath. The intermittent event starts on DOY 178 in 2014 upstream of the shock wave that overtook V1 on DOY 237 at 140 au from the Sun. This turbulence enhancement coincides with the recovery in the Galactic cosmic-ray flux perpendicular to B , which further results in the flux isotropization.

Here, the distribution of magnetic field increments follows the q-Gaussian distribution with the index q=1.57 for the maximum variance component. Evidence is provided of fine-scale structures well below the Coulomb collisional scale. They undergo a turbulent cascade at scales less than 10^-3 au. Observed magnetohydrodynamic fluctuations show signatures of irregular filamentary structures, sawtooth-like waveforms of mixed compressible/transverse nature, which evolve into shocklets and current sheets. Observational data challenges the interpretation of the interstellar plasma in the outer heliosheath as a featureless medium at scales below the collisional mean free path of about 1 au. Moreover, we discuss the spectral properties and evolution of magnetic field fluctuations in the outer heliosheath over seven-year of V1 observations, till 2019 (144 au).

These results are of importance for better understanding of turbulent processes in the outer heliosheath, including magnetic reconnection, shock-turbulence interaction, Galactic cosmic-ray transport, plasma wave generation, and radio emission.