No Stagnation Region before the Heliopause at Voyager 1? Inferences from New Voyager 2 Results

Abstract

We present anisotropy results for anomalous cosmic-ray protons in the energy range ~0.5–35 MeV from Cosmic Ray Subsystem (CRS) data collected during calibration roll maneuvers for the magnetometer instrument when Voyager 2 (V2) was in the inner heliosheath. We use a new technique to derive for the first time the radial component of the anisotropy vector from CRS data. We find that the CRS-derived radial solar wind speeds, when converted from the radial components of the anisotropy vectors via the Compton–Getting (C–G) effect, generally agree with those similarly derived speeds from the Low Energy Charged Particle experiment using 28–43 keV data. However, they often differ significantly from the radial solar wind speeds measured directly by the Plasma Science (PLS) instrument. There are both periods when the C–G-derived radial solar wind speeds are significantly higher than those measured by PLS and times when they are significantly lower. The differences are not expected nor explained, but it appears that after a few years in the heliosheath the V2 radial solar wind speeds derived from the C–G method underestimate the true speeds as the spacecraft approaches the heliopause. We discuss the implications of this observation for the stagnation region reported along the Voyager 1 trajectory as it approached the heliopause inferred using the C–G method.

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