Voyager 1 in the Foreshock, Termination Shock, and Heliosheath

Abstract

Voyager 1 (V1) began measuring precursor energetic ions and electrons from the heliospheric termination shock (TS) in July 2002. During the ensuing 2.5 years, average particle intensities rose as V1 penetrated deeper into the energetic particle foreshock of the TS. Throughout 2004, V1 observed even larger, fluctuating intensities of ions from 40 kiloelectron volts (keV) to ≥50 megaelectron volts per nucleon and of electrons from >26 keV to ≥350 keV. On day 350 of 2004 (2004/350), V1 observed an intensity spike of ions and electrons that was followed by a sustained factor of 10 increase at the lowest energies and lesser increases at higher energies, larger than any intensities since V1 was at 15 astronomical units in 1982. The estimated solar wind radial flow speed was positive (outward) at ∼+100 kilometers per second (km s –1 ) from 2004/352 until 2005/018, when the radial flows became predominantly negative (sunward) and fluctuated between ∼–50 and 0 km s –1 until about 2005/110; they then became more positive, with recent values (2005/179) of ∼+50 km s –1 . The energetic proton spectrum averaged over the postshock period is apparently dominated by strongly heated interstellar pickup ions. We interpret these observations as evidence that V1 was crossed by the TS on 2004/351 (during a tracking gap) at 94.0 astronomical units, evidently as the shock was moving radially inward in response to decreasing solar wind ram pressure, and that V1 has remained in the heliosheath until at least mid-2005.