Zero outward flow velocity for plasma in a heliosheath transition layer

摘要

Voyager 1 has been in the reservoir of energetic ions and electrons that constitutes the heliosheath since it crossed the solar wind termination shock~(1-3) on 16 December 2004 at a distance from the Sun of 94 astronomical units (1 au = @@1.5 X 10~8 km). It is now ~22 au past the termination shock crossing~4. The bulk velocity of the plasma in the radial-transverse plane has been determined5 using measurements of the anisotropy of the convected energetic ion distribution~6. Here we report that the radial component of the velocity has been decreasing almost linearly over the past three years, from ~70kms~(-1) to ~0kms~(-1), where it has remained for the past eight months. It now seems that Voyager 1 has entered a finite transition layer of zero-radial-velocity plasma flow, indicating that the spacecraft may be close to the heliopause, the border between the heliosheath and the interstellar plasma. The existence of a flow transition layer in the heliosheath contradicts current predictions~7-generally assumed by conceptual models-of a sharp discontinuity at the heliopause.