A New Look at the Heliosphere and Solar Modulation

1. Cosmic Ray Transport In The Heliosheath The View From Voyager A New Look at the Heliosphere and Solar Modulation F.B. McDonald1, A.C. Cummings2, B.C. Heikkila3, N. Lal3, E.C. Stone2, 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 ISSI Workshop April 12-16, 2010 Bern, Switzerland

2. Galactic Cosmic Rays in the Heliosheath • Voyager 1 crossed the TS and entered the region of the heliosheath on 2004.96 [94 AU, 34°N] • Voyager 2 followed on 2007.67 [83.7 AU, 27.5°S] • GCR Electrons: 2.5 – 160 MeV • 2.5 – 30 MeV E observed as V1 and V2 approached the TS. Generally associated with the passage of interplanetary transients • At V1 large continuing increases in all electron channels (currently ~80% / yr for 10 MeV E) • The V2 time-history suggests electron reacceleration at the TS for 2.5 – 30 MeV E. • The V2 time histories in the heliosheath are very different from those of V1. • GCR Ions • Significant on-going increase for 150-380 MeV/n GCR He (currently 7.4% / Year; 2006.14-2009.2) • and 175-345 MeV GCR H (15.4 % / Year) • The radial intensity gradient is < 0.2  0.3% / AU • This increase is temporal – not spatial

3. Low Energy Galactic Cosmic Ray Electrons (2 – 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.

4. Low Energy Galactic Cosmic Ray Electrons (2 – 160 MeV) : A New Ultra-Relativistic, Very Low Rigidity Tool for Modulation Studies The Voyager Electron Telescope Electron Telescope Calibration

5. Low Energy Galactic Cosmic Ray Electrons (2 – 160 MeV)(Raw Data) • Data from 1997.0 – 2002.0 indicates the background level of the electron telescope. • There is no radial intensity gradient despite the large radial separation. • The background is produced by higher energy protons (close to minimum ionizing)

6. Low Energy Galactic Cosmic Ray Electrons (2 – 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.

7. V1,V2 Background Subtracted Low Energy Galactic Cosmic Ray Electrons The large electron increases that occur just prior to the V1 TSX are associated with the passage of strong IP transients. The V2 TSX occurred closer to solar minimum and the large increases at energies less than 30 MeV immediately after crossing are interpreted as local acceleration at the TS.

8. Voyager-2 Electron Data Time Shifted so that the Time of TSX Coincides with that of Voyager-1 (Background Subtracted) • There is reasonable correspondence between the Voyager 1, Voyager 2 intensities after 2006 (V1 time) in the vicinity of the TS. • Note the absence of electron increases at energies > 30 MeV associated with the Voyager 2 termination shock crossing.

9. Voyager Electron Energy Spectra • The V2 electron spectra is from shortly after the termination shock crossing. • The V2 spectra is steeper than that of V1 which is interpreted as due to reacceleration at the termination shock. • V1 spectral slope = 1.6 • V2 spectral slope = 2.9

10. Voyager Electron Energy Spectra • The V2 electron spectra is from shortly after the termination shock crossing. • The V2 spectra is steeper than that of V1 which is interpreted as due to reacceleration at the termination shock. • V1 spectral slope = 1.6 • V2 spectral slope = 2.9

11. Electron Transients and Electron Acceleration at the Termination Shock Steep V2 energy spectra and large increases down stream from the TS are suggestive of a local acceleration process (TS?) • 1st electron increases since 1985 except for increases associated with GMIRS produced by the Sep, Oct 1989 and Jun 1991 flares/CMEs. • Transient IP disturbances defined by HEP (H > 200 MeV) decreases. • Good correlation between 10 MeV electron and 22 MeV H increases and HEP increases.

12. 10 MeV Electrons and 24 MeV H respond to the same transients. • Transients appear to be confined to the inner heliosheath.

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

14. Cosmic Ray Modulation

15. Cosmic Ray Modulation

16. Cosmic Ray Modulation Cycle 23 Voyager Data 01/01-02/22 / 2006-2010 (52 Day Periods) 07/02-08/23 / 2006-2010 (52 Day Periods)

17. Cosmic Ray Modulation Cycle 23 Voyager Data 01/01-02/22 / 2006-2010 (52 Day Periods) 07/02-08/23 / 2006-2010 (52 Day Periods)

18. GCR He and H in the Heliosheath • Radial Gradients < 0.2 %/AU over • 2005-2009.2 • The V2 decrease starting in 2009.2 • is not understood

19. Voyager-1 2006.14 – 2008.92 % Below Webber/Higbie LIS 150 - 380 MeV/n GCR He 7.4 %/Year 21% 145 – 244 MeV/n GCR He 9.6 %/Year 30% 180 - 350 MeV GCR H 15.5 %/Year 44% 30 – 56 MeV/n ACR He 3.5 %/Year 6 - 14 MeV GCR E 75.0 %/Year

20. Regression

21. Cosmic Ray Modulation

22. Webber, 2010

23. What is Happening at 1 AU 6

24. Model Heliosphere • 10 MeV Galactic Cosmic Ray Electrons are increasing at a rate of 75%/yr. • No radial intensity gradient for 150-380 MeV/n GCR Helium (0.1 ± 0.2%/AU) between V1 and V2. • Strong correlation between intensity increases of Cosmic Ray 10 MeV Electrons and 265 MeV/n He. • There is expected to be a strong magnetic barrier at the front and flanks of the heliopause (The Axford-Cranfield effect). • Two Possible Solutions • The heliopause is highly permeable to energetic particles for reasons that are not understood at this time • The GCR Electrons and Medium energy ions (< 350 MeV/n) gain entry into the heliosheath and the heliosphere through the heliotail region. Stay Tuned.

25. Discussion and Conclusions • The ongoing period of very low solar activity has a multi-faceted effect on the modulation process. • The increase in 135 - 250 MeV/n GCR He at 1 AU appears to be strongly related to the decrease in the heliospheric magnetic field. • The tilt angle changes of the heliospheric current sheet plays a major role for ACR O and for Neutron Monitors at 1 AU. • The lower IPB field will have a significant effect through-out the heliosphere. At the Voyagers in the heliosheath there is an on-going increase of 265 MeV/n He (7.4%/year) and H (15.5%/year) that is temporal and not spatial in nature. Since drift effects are not expected to be important in the heliosheath, this increase is probably related to the lower B field. • The lower solar wind velocity and pressure will have an effect on the dimensions of the heliosphere and probably on the particle diffusion coefficients. • Over the last 1000 years there have been previous epochs of low solar activity that have resulted in significant increases in the GCR intensity. As measured by archival data from 10Be in polar ice cores and 10C in tree rings. • The current Quiet Sun period should provide insight into the changes that occurred on the Sun and in the IP medium over those very unusual earlier periods. It is especially important to get contemporaneous measurements of 10Be.