Zero outward flow velocity for plasma in a heliosheath transition layer

Abstract

Voyager 1 has been in the reservoir of energetic ions and electrons that constitutes the heliosheath since it crossed the solar wind termination shock^1,2,3 on 16 December 2004 at a distance from the Sun of 94 astronomical units (1 au = 1.5 × 10⁸ km). It is now ∼22 au past the termination shock crossing⁴. The bulk velocity of the plasma in the radial–transverse plane has been determined⁵ using measurements of the anisotropy of the convected energetic ion distribution⁶. Here we report that the radial component of the velocity has been decreasing almost linearly over the past three years, from ∼70 km s⁻¹ to ∼0 km s⁻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 predictions⁷—generally assumed by conceptual models—of a sharp discontinuity at the heliopause.