Energetic particle variations measured at Voyager 1 and 2 in 2013-14

Abstract:

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.