Waves and Turbulence in the Very Local Interstellar Medium: From Macroscales to Microscales

Abstract

Abstract Voyager 1 (V1) has been exploring the heliospheric boundary layer in the very local interstellar medium (VLISM) since 2012 August. The measurements revealed a spectrum of fluctuations over a vast range of space and timescales, but the nature of these fluctuations continues to be an intriguing question. Numerous manifestations of turbulence cannot be explained using a single phenomenology. Weak shocks and compressions are the prominent features of the VLISM. We use high-resolution (48 s) measurements to perform a multiscale analysis of turbulence at V1 between the years of 2013.36 and 2019.0 (124–144 au from the Sun). On relatively large scales, wave trains of mixed compressible/transverse nature with the correlation scale in the range of 15–100 days dominate the spectrum of fluctuations. The observed magnetic field profiles are suggestive of a Burgers-like (f −2) turbulence phenomenology induced by solar activity. We demonstrate that the level of large-scale compressible fluctuations is still significant in late 2018. We analyze the turbulence down to small scales comparable to the ion inertial length and show that magnetic compressibility is always large on these scales. Besides the shock-induced turbulence measured from 2014.486, the intensity and intermittency of small-scale fluctuations have been growing smoothly since 2018.5. Our analysis suggests that local processes are contributing to the production of turbulence on small scales. We present the estimates of transport coefficients in the plasma traversed by V1. The range of scales is identified where V1 measurements are affected by the contribution from pickup ions.