Energetic particle variations measured at Voyager 1 and 2 in 2013-14
Monday, 15 December 2014: 2:40 PM
In late August of 2012 Voyager 1 evidently crossed the heliopause at 121.6 AU near the nose of the heliosphere and entered the local interstellar medium (LISM). Since that time Voyager 1 has been in a relatively stable, but not steady-state region. Low-energy ion and electron intensities measured by the LECP instrument on Voyager 1 continue to be down by factors of 103 to 104 for major ion species compared to those in the heliosheath, with no evidence of anomalous cosmic rays upstream. The anisotropy of galactic cosmic ray protons >211 MeV, which reached a maximum ≈9% in April 2013, persisted for about one year after Voyager 1 entered the LISM, suggesting a transition region upstream of the heliopause of ≈4 AU. However, the increase in anisotropy has resumed, suggesting that influence of the heliosphere persists to distances ≈7 AU. In addition, small (≈1%) increases in the angular-averaged GCR proton intensities, the most recent occurring in mid-April of 2014 (at 127.5 AU), have been attributed to large disturbances due to solar activity and are also associated with activity in the Voyager 1 Plasma Wave instrument [Gurnett et al. 2014, this session]. Voyager 2 is now at 106 AU and still firmly in the heliosheath, with the lower-energy ion intensities having increased by a factor ≈3 since reaching a minimum in early 2013. Although the intensities of low-energy heliosheath ions and electrons continue to increase, they remain variable on short time scales. The spectral hardening of low-energy ions observed at Voyager 1 beginning about ≈1.5 years before its crossing of the heliopause is totally absent at Voyager 2. Hence, it appears highly unlikely that Voyager 2 is approaching the heliopause in the near future.