Robert B. Decker
Johns Hopkins Applied Physics Laboratory
Voyager 2 at the Termination Shock: Energetic Particle Perspective
Broad regions on both sides of the solar wind termination shock are populated by high intensities of non-thermal ions and electrons. The pre-shock particles in the solar wind were measured at Voyager 1 from mid-2002 through Dec. 2004 and at Voyager 2 from early 2005 through mid- 2007. The post-shock particles in the inner heliosheath have also been measured at Voyager 1 since it crossed the termination shock in Dec. 2004. However, it had not been clear what effect these particles might have on the physics of the shock transition until Voyager 2 crossed the shock during days 242-245 of 2007. Unlike Voyager 1, Voyager 2 is making plasma measurements. Data from the plasma and magnetic field instruments on Voyager 2 indicate that non-thermal ion distributions likely play key roles in mediating dynamical processes at the termination shock and in the inner heliosheath. Intensities of low-energy ions measured at Voyager 2 produce non-thermal partial ion pressures in the heliosheath that are comparable to (or exceed) both the thermal plasma pressures and the scalar magnetic field pressures. These ions are evidently the >0.028 MeV portion of the non-thermal ion distribution that determines termination shock structure and whose acceleration extracts a large fraction of bulk flow kinetic energy from the incident solar wind. We discuss these results, compare energetic particle data from the two Voyagers, and summarize recent observations.