Mediation of the solar wind termination shock by non-thermal ions

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 have been measured by the spacecraft Voyager 1 ( refs 1 - 5) and Voyager 2 ( refs 3, 6). The post- shock particles in the heliosheath have also been measured by Voyager 1 ( refs 3 - 5). It was not clear, however, what effect these particles might have on the physics of the shock transition until Voyager 2 crossed the shock on 31 August - 1 September 2007 ( refs 7 - 9). Unlike Voyager 1, Voyager 2 is making plasma measurements(7). Data from the plasma(7) and magnetic field(8) instruments on Voyager 2 indicate that non- thermal ion distributions probably have key roles in mediating dynamical processes at the termination shock and in the heliosheath. Here we report that intensities of low- energy ions measured by 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. We conclude that these ions are the >0.028 MeV portion of the non- thermal ion distribution that determines the termination shock structure(8) and the acceleration of which extracts a large fraction of bulk- flow kinetic energy from the incident solar wind(7).