Turbulence in the Outer Heliosheath

Abstract

Abstract We present in situ observations of magnetic turbulence in the draped interstellar magnetic field measured by Voyager 1 during an undisturbed interval from 2015.3987 to 2016.6759 confirming the existence of the turbulence observed previously from 2013.3593 to 2014.6373. The power spectral density of the turbulence was the same in both cases. The turbulence had a Kolmogorov k −5/3 spectrum in the range from k = 1.3 × 10−13 cm−1 to 4 × 10−12 cm−1. The ratio of the turbulent fluctuations to the average magnetic field strength was only 0.02, indicating that the turbulence was very weak. Extrapolating the power-law slope to lower frequencies yields an upper limit on the turbulence outer scale of 0.01 pc = 2000 au, which may be regarded as the distance at which Voyager 1 will enter the undisturbed local interstellar medium, beyond the outer heliosheath or bow wave in the upstream direction. The maximum variance of the fluctuations was in the two directions transverse to the average magnetic field in the recent interval, whereas it was parallel to the average magnetic field in the earlier interval, suggesting a transformation from turbulence with a dominant compressive component to turbulence dominated by transverse fluctuations. As the magnitude of the fluctuations was approaching that of the uncertainties of the measurements, the latter result requires confirmation by further observations.