Observations of Pick-up Ions in the Outer Heliosphere by Voyagers 1 and 2

S.M. Krimigis, R.B. Decker
	Applied Physics Laboratory, Laurel, Maryland, USA
D.C. Hamilton, G. Gloeckler
	University of Maryland, College Park, Maryland, USA

Observations from the Low Energy Charged Particle (LECP) instrument on the Voyager 1 (V1) spacecraft at ~70AU during the last solar minimum revealed an excess of counts from the sunward direction in the nominal proton energy range ~40 to ~140keV. This, above background, response was also seen by Voyager 2 at ~55AU and was present during the previous solar minimum at V1 at ~28AU. The angular distribution is inconsistent with these counts being due to hot protons convected into the sunward-viewing sector only. We have examined possible sources for the observed counts and have considered that they may be due to singly-ionized heavy (A> or equal to 16) ions picked up by the solar wind and convected into the sunward sector of the detector. The spectrum is steep (dj/dE ľ E-6), and exhibits a cutoff at ~20keV/nuc. This observation suggests that pickup ions may be present beyond 5 AU, and could be the pre-accelerated seed population for anomalous cosmic rays (ACR).