An energetic-particle-mediated termination shock observed by Voyager 2

Abstract

Voyager 2 crossed the solar wind termination shock several times during August 30 - September 1 of 2007. During the last forty days before the crossing intensities of energetic ions measured by the LECP instrument were increasing exponentially, and their spectrum showed clear evidence of unfolding at low energies. Plasma data featured a broad velocity precursor, where solar wind speed decreased from about 380 km/s far upstream to ∼300 km/s at the shock. By using plasma and energetic particle conservation laws we demonstrate that the speed decrease during Voyager 2's last ∼40 days in the solar wind could be plausibly produced by the back-pressure of energetic ions with energies of a few MeV on the upstream plasma flow. These particles propagate diffusively upstream from the termination shock, producing a shock precursor that has a characteristic lengthscale of 0.35 AU, assuming the shock has not moved substantially during that time. Copyright 2009 by the American Geophysical Union.