Johns Hopkins University Applied Physics Laboratory
Low-energy Charged Particles at Voyagers 1 and 2 in the Heliosheath
Voyager 1 (115 AU, N34 deg.) and Voyager 2 (93 AU, S29 deg.) have been in the heliosheath since Dec. 2004 and Sep. 2007, respectively. Intensities of low-energy ions 40 keV - few MeV measured by the LECP instrument on Voyager 1 have been relatively steady since mid-2006. The heliosheath-averaged (5.6-year) Voyager 1 energy spectrum rolls over slightly with increasing energy, with the spectral index changing from -1.5 to -1.7. Intensities of ions 30 keV - few MeV observed by the LECP instrument on Voyager 2 have been relatively steady since 2009.4, in contrast to the large quasi-recurrent variations observed from 2007.66 (termination shock) to 2009.4. The low-energy ion energy spectrum at Voyager 2 is slightly harder than that at Voyager 1. The Voyager 2 spectrum averaged over 2007.66-2009.4 shows a rollover, with the spectral index changing from -1.3 to -1.5. Intensities of energetic (0.02-1.5 MeV) heliosheath electrons began decreasing exponentially at Voyager 2 on 2009.15, and are now at detection background. Energetic electron intensities at Voyager 1 remain high. We estimate the heliosheath plasma flow velocity in the R-T (LECP instrument scan) plane at Voyager 1 by using angular data from the three lowest energy ion channels that cover 40-139 keV. LECP cannot measure the normal component of flow WN. During 2007.7-2010.7, the estimated radial component of flow WR decreased from ~ 60 km/s to ~ 0 km/s at a rate of -20 km/s/yr. On average, the tangential component of flow WT has been ~ -40 km/s from termination shock crossing onward, but continues to show broad variations ±20 km/s.