Robert B. Decker
Johns Hopkins University Applied Physics Laboratory
The Voyager Spacecraft at the Termination Shock
It has been nearly a year since Voyager 1 encountered the solar wind termination shock on or about day 351 of 2004 (94.0 AU, N34 deg.) and entered the heliosheath. Since that time until at least day 320 of 2005 (97.3 AU), observations show that the spacecraft has remained in the heliosheath. We will describe intensities and angular distributions of ions >40 keV and electrons >26 keV associated with the termination foreshock, termination shock, and heliosheath. These measurements are from the Low Energy Charged Particle instruments on Voyagers 1 and 2. Notable features observed thus far in the heliosheath particle data from Voyager 1 include: (1) high, slowly rising, relatively smooth intensities devoid of the large (factors ~5-10), frequent (~26 days) fluctuations that characterized intensities in the termination foreshock; (2) large reductions in the amplitudes and occurrence rates of the anti-sunward, near-azimuthal beaming anisotropies that were routinely seen in the foreshock ion data; and, (3) radial plasma flow speeds, estimated from analysis of low-energy ion angular distributions, that remain less than ~100 km/s. Recent data show relatively rapid (~15 days) increases of ion and electron intensities, but no marked increases in anisotropies or deduced radial flow speeds. We are fortunate to be currently receiving heliosheath data from Voyager 1 and solar wind data from Voyager 2, which is evidently now in termination foreshock region. Voyager 2 began observing low intensities of termination shock precursor protons in late 2004 (75 AU, S26 deg.). By mid-2005 (77 AU), the intensity of 3-17 MeV protons measured at Voyager 2 reached levels comparable to those observed at Voyager 1 during the latter half of 2002 (85-87 AU). Ion data at Voyager 2 show quasi-recurrent (~26 days) intensity variations and near-azimuthal beaming anisotropies that are directed predominantly sunward, opposite to the predominantly anti-sunward beaming anisotropies that were measured in the termination foreshock by Voyager 1.