Voyager SSG November 3-4, 2011

1. Voyager Observations of Cosmic Rays in the Heliosheath F.B. McDonald1, A.C. Cummings2, E.C. Stone2 , B.C. Heikkila3, N. Lal3, W.R. Webber4 1 Institute for Physical Science and Technology, Univ. of Maryland, College Park, MD, USA 2 California Institute of Technology, Pasadena, CA, USA 3 NASA/Goddard Space Flight Center, Greenbelt, MD, USA 4 Dept. of Physics and Astronomy, New Mexico State Univ., Las Cruces, New Mexico, USA Voyager SSG November 3-4, 2011

2. Voyager Cosmic Ray Telescopes Energetic Particle Coverage H: 1.8-300 MeV He: 1.8-650 MeV/n Z = 1-28 (Resolves Isotopes) E: 2.5 – 160 MeV ← 28.6 cm → • Outline • Introduction • What’s up at 1 AU! • Galactic Cosmic Rays in the Heliosheath. • Long-lasting Heliosheath Intensity Increases • Discussion

3. Introduction • Voyager observations in the heliosheath now extend from the solar maximum and solar minimum epochs of cycle 23 and well into the onset of cycle 24. • At 1 AU the cycle 23 solar minimum was the longest and deepest of the last ~ 100 years and cycle 24 so far has been weird in its own special way. • The combined Pioneer-Voyager-IMP-ACE solar minimum/solar maximum data indicate the GCR 265 MeV/n He intensities did not change significantly at the location of the termination shock from 1997/1998 through 2003. However at V1 there was a rapid increase associated with the onset of recovery from the cycle 23 solar maximum. • The time history of GCRs in the heliosheath at V1 is relatively simple and is dominated by 2 long-term exponential increases. At V2, these time histories are more complex. May reflect the different phase of the solar cycle and the relative penetration depth into the heliosheath. • Long lasting heliosheath intensity increases play a major role at V2. Similar increases were observed at both V1 and V2 as they approached the TS. • There is a marked change in the GCR response to these transients as V2 moves deeper into the heliosheath. Are there essentially simultaneous increases at V1 and 2? • Definitions: • Dynamic Gradient: GDYN • Intensity increase / unit time divided change in radial distance over this period • Intensity Gradient: 1/r • (dJ/dr) = gr

4. IPB is 28% below the previous minima of cycles 19-22. Heliospheric neutral current sheet approaches minimum value at a slower rate than previously observed. Wang, Robbrecht and Sheely (2009) have pointed out that these effects appear to be related – the weaker polar fields result in the apparent refusal of the heliospheric current sheet to flatten closer to the equator.

5. 2006.75 – 2011.0 a) 6.4% / year (V1) GD = 1.8 % / AU (2010.25) gr = 0.6 % / year b) 11.6 % / year (V1) GD = 3.2 % / AU (2010.0) gr = 1.2 % / year The “decrease in the V2 intensities between 2009.25 – 2011 is an artifact created by the long lasting transient increases peaking in late 2008 and 2009. There is a significant radial gradient for GCR H and He.

6. 2006.75 – 2011.0: 60 % / year (V1) GD = 16.6 % / AU (2010.0) gr = 14.6 % / year

12. 1 AU Comparison

13. GCR Electrons 2.7-160 MeV Origin: • Knock-on electrons • Directly accelerated primaries • Interstellar secondaries from the decay of charged pions The interstellar spectrum can be determined directly from the shape of the VLF galactic polar radio spectrum via synchrotron formula for electron motion in the galactic magnetic fields Webber and Higbie have recalculated the interstellar electron spectrum based on work by Vladimir Ptuskinet al, 2006 on the diffusion of galactic cosmic rays in the interstellar medium

16. Electron Spectra

18. HELIOSHEATHTRANSIENT INCREASES

19. Arrows define discontinuous changes in the 10MeV electron intensity. Note that the rate of increase appears to be approximately the same in alternate regions.

20. Arrows define discontinuous changes in the 10MeV electron intensity. Note that the rate of increase appears to be approximately the same in alternate regions. Stone, et. al,., 2011; Burlaga and Ness, 2010

21. Arrows Mark Voyager-1 Events

25. Discussion • At V1 significant variability in all components from TSX to ~ 2008.25. Then relatively smooth, exponential increases in GCRs • Long lived TSP events previously noted as V1 (and V2) neared the TS. They appear to represent periods when the spacecraft is magnetically connected to the TS. • The 2 modest jumps in V1 electron intensity at 2009.7 and 2011.25 represent passage into a different magnet field regime. • The V2 long lasting, transient increases probably also reflect connections to a different regime of the heliosheath. • The C low energy J=AT slope is consistent with the concept advanced by Florinski and Pogorelov (2008) that the heliosheath is a long term storage region for GCRs < ~ 200MeV/n. • The flat time history of V1 GCR He > 265 MeV/n and the decrease in V1 ACR and TSP intensities may suggest V1 is approaching the heliopause. • The End Is Near!!!