39 th COSPAR Scientific Assembly Mysore, INDIA July 14-22, 2012

1. Voyager In The Distant Heliosheath: Continuing The Quest Of Victor Hess F.B. McDonald1, W.R. Webber2,E.C. Stone3, A.C. Cummings3, B.C. Heikkila4, N. Lal4 1 Institute for Physical Science and Technology, Univ. of Maryland, College Park, MD, USA 2 Dept. of Physics and Astronomy, New Mexico State Univ., Las Cruces, New Mexico, USA 3 California Institute of Technology, Pasadena, CA, USA 4 NASA/Goddard Space Flight Center, Greenbelt, MD, USA 39th COSPAR Scientific Assembly Mysore, INDIA July 14-22, 2012

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 Cosmic Ray Modulation: 1 AU to the Termination Shock The Pioneer 10/11, Voyager 1/2 Story Voyager Electron Studies Galactic Cosmic Rays in the Heliosheath and the Recent Voyager 1 GCR Enhancement Event Voyager 2 Long-lasting transient increases: The Sector Zone

3. Galactic Cosmic Ray Modulation • Probes the large scale structure and dynamics of our heliosphere • CR Modulation is dominated by the Sun • Level of solar activity – (CME’s) • Tilt angle of the neutral HCS • Velocity (and density) of Solar Wind • Strength of IP magnetic field • Physical Processes • Diffusion • Convection • Adiabatic Energy Losses • Drifts 10 MeV 1 GeV • Over the modern era (1951 – present) GCR intensity is at its lowest level over the past 1000 years. • Modulation studies will allow the exploration of the role of these different phenomena.

4. Galactic Cosmic Ray Modulation • V1, V2 in the heliosheath will provide valuable insight into what is happening! • The deep solar minimum of cycle 23/24 provided an unprecedented opportunity for modulation studies: • To quantify the effects of reducing the strength of the Interplanetary Magnetic Field and of transit changes in the current sheet tilt angle. • Better understanding of unusual epochs in the past such as the Sporer and Maunder minima. • Time Scales • 11 year solar activity – dominated by level of solar activity – effects of ICMEs (reasonable correlation with sunspot numbers) • Heliomagnetic (22 years): very different time histories at 1 AU for qA> 0 minima (when particles drift in over the solar poles and out along the current sheet. Flow pattern reversed for qA<0 (odd cycle) • q A<0 at 1 AU enhancement of GCRs > ~ 1 GV • Suppression of GCRs and ACRs < 1 GV

5. An Overview of GCR He Modulation in the HeliosphereOver the Past 39 Years • Concentrate on Solar Minimum/Maximum Periods

6. Cosmic Ray Modulation

7. Cosmic Ray Modulation

8. Cosmic Ray Modulation Cycle 23 Voyager Data 01/01-02/22 / 2006-2012 07/02-08/23 / 2006-2012 Similar behavior for GCR H and ACR O in qA > 0 epochs Major change in modulation conditions between ~12 AU (where merged interaction regions form) and the termination shock GCR and ACR intensities are constant from solar minimum to solar maximum at the termination shock near the equatorial plane

9. Low Energy GCR Electrons (2.5 – 160 MeV): A New Ultra-Relativistic, Very Low Rigidity Tool for Modulation Studies • Origin: • Directly accelerated primaries • Interstellar secondaries from the decay of charged pions • Knock-on electrons produced by the passage of higher energy cosmic rays through the interstellar medium • At energies below 200 MeV: • These electrons are the source of the lower energy diffuse gamma, x-ray and synchrotron radio emission from the galaxy. • May play a major role in ionizing and heating the interstellar medium. • Difficult to observe at 1AU: • Large Jovian electron intensity • Strongly modulated • In the heliosheath, their very low rigidity should make them especially sensitive to the passage of transient disturbances.

10. Low Energy GCR Electrons (2.5 – 160 MeV) : A New Ultra-Relativistic, Very Low Rigidity Tool for Modulation Studies The Voyager Electron Telescope Electron Telescope Calibration

11. Low Energy Galactic Cosmic Ray Electrons (2.5 – 160 MeV) versus HEP • Background correction procedure for selected electron channels. HEP rates correspond to H > 200 MeV • The electron intensity closely tracks the integral rate of GCR H > 200 MeV from 1986 to 2002. In addition, the electron “radial intensity gradient” was negative in 1987 between V1 and V2, clearly a background effect. The detector is background dominated until it reaches the vicinity of the heliosheath.

13. 2006 – 2011 V1: 6.8% / Yr GD = 1.9% / AU gr = 0.6% / Yr V1: 13.1% / Yr GD = 3.6% / AU gr = 1.2% / Yr V1: 19.3% / Yr GD = 5.4% / AU V1: 61.8% / Yr GD = 17.2% / AU gr = 14.6% / Yr

15. GCR Helium Spectra

18. Voyager 2 Long – Lasting Transient Increases • These increases are different from the recently observed V1 enhancement. There is a close correlation between ACRs and GCRs as will be seen in the next slide. • The solid bar between event 2 and 3 represents passage into a unipolar region of space (Burlaga and Ness, ApJ 2011) • Close correlation between increases of low energy particles at V2 and passage in and out of the sector zone first noted by LECP experiment.

19. Voyager 2 Long-Lasting Transient Increases I II III

22. Discussion • The V2 large scale intensity changes on crossing into the sector zone suggest this could be the primary entry from interstellar space into the heliosheath. • The V1/V2 GCR intensities are not affected by the changes in the solar wind speed and direction in the heliosheath. • The V1 GCR Enhancement event (ACR decrease) suggests that we are near the heliopause. • The continuing increase in low energy GCR electrons indicates that all GCRs have access to the heliosheath. • The ACRs may be the best cosmic ray monitor for the heliopause crossing. • The close agreement with the Webber-Higbie LIS is another strong indication that V1 is approaching the heliopause.

23. Are we there yet? • Yes • No • Getting Close X