Matthew E. Hill
Johns Hopkins University Applied Physics Laboratory
Puzzling out the Heliosheath Observations from the Voyager Probes
The nature of the exploration of the heliosheath by Voyager 1 and 2 does not lend itself to definitive observations and rapidly converging interpretations; the region is so immense (millions of cubic AU) and the spacecraft, traveling a few AU per year, return but threads of observational evidence as they pierce the veil. But how tantalizing the observations are. We are now in an exciting period of the mission during which new, brow-furrowing features seem to arise every few months and expectations have short lifetimes. I will first review the latest measurements from a number of the instrument teams. For example, the plasma measurements at both spacecraft are unprecedented. At Voyager 1 the ubiquitous radial flow of solar wind (~400s of km/s inside the termination shock) has stopped and is now actually seen to be flowing back toward the Sun at roughly 20 km/s. At Voyager 2 the plasma has made "a hard left turn" and is now flowing ~60 degrees away from radial down towards the heliosphere’s tail, and the density is increasing. There are similarly striking observations of "termination shock particles" (ions with energies in the 100's of keV), anomalous cosmic rays, (ions up to ~100 MeV), galactic cosmic rays, relativistic electrons, and the magnetic field. After this I will discuss evidence that the region of the heliosheath enveloping the heliospheric current sheet (sometimes called the sector zone) is very different from the regions of single magnetic polarity above and below the sector zone. It is the low-latitude sector zone where ions from 10s of keV to 100s of MeV and electrons from 10s of keV to 10s of MeV are efficiently transported (or perhaps accelerated) into and throughout the heliosphere and therefore understanding this zone is fundamental to understanding the physics of the heliosheath and indeed the heliosphere as a whole.