Scientific Colloquium
May 7, 2004
                "Voyager at the Edge of the Solar System?"
The Voyager 1 magnetic field observations do not support the view that Voyager 1 passed from the solar wind to a subsonic region such as the heliosheath and back to the solar wind near 85 AU during 2002. The magnetic field measured by Voyager 1 has the expected properties for the heliospheric magnetic field at that distance and epoch of the solar cycle. The relation between the V1 magnetic field strength variations and the cosmic ray variations during that interval is also consistent with previous observations in the solar wind. Results of a search for a transition from solar wind to the heliosheath during 2003 will be discussed.

We report analyses of a large increase in the intensity of energetic particles observed with the Low Energy Charged Particle (LECP) experiment on Voyager 1 (V1) in the outer heliosphere. We argue that V1 exited the solar wind and passed into a new region (possibly beyond the TS) about August 1, 2002 at a distance of ~85 AU (heliolatitude ~34° N), then re-entered the solar wind about 200 days later at ~87 AU. We use LECP data to infer that the solar wind became subsonic. The LECP data also show the composition of ions accelerated at the putative TS to be that of anomalous cosmic rays (ACR) and of interstellar pickup ions (PUI). We will also report on ongoing activity at V1 that shows similar characteristics to the earlier period, now near 92 AU.

In late 2003 as Voyager 1 moves beyond 90 AU in the recovery phase of solar cycle 23, the effects of the termination shock (TS) and the heliosheath on particle transport are becoming more evident.  There are now detectable fluxes of 2.7 – 70 MeV electrons but at very low intensity levels that suggest they are heavily modulated in the heliosheath.  The modulation of galactic and anomalous cosmic rays is significant but much smaller than observed at 1 AU.  At V1 the first large increase of MeV ions was observed and lasted for 6.5 months.  These ions appear to originate at the TS. And a second very similar event has now been in progress for some eight months.  It is argued that these are the type of precursor events that are expected as Voyager 1 approaches the termination shock.  At V2, 17 AU behind V1, there are a series of 8 increases of low energy ions that occur approximately every 150 days starting in late 2000.  Many of these increases can be related back some 6 months prior to specific periods of high solar activity.