Outline Our place in the Galaxy The Local Interstellar Cloud (LIC)

1. The Voyagers at the Edge of the Heliospheric BubbleJohn D. Richardson (MIT)With thanks to the Voyager team and E. Mobius Outline Our place in the Galaxy The Local Interstellar Cloud (LIC) The Solar Wind - LIC interaction The Voyagers at the Termination Shock The Future

2. Our ``County‘‘ in the Cosmos: The Local Bubble Local Bubble and Loop I are Interacting Bubbles! Sun is inside hot local bubble formed by supernova explosions. The bubble has small denser cooler clouds, perhaps breaking off bubble boundaries. The Sun is in one of these clouds. Blasts from Past ISM Cloud Flow From ISSI Workshop: From the Heliosphere to the Local Bubble Sticking our Head out E. Möbius UNH/SSC

3. Apropos: Sticking Our Head Out Heliosphere: pressure balance between solar wind and local interstellar medium. Magnetized plasmas cannot mix. Boundary is the Heliopause Shocks form in both flows, plasma moves downstream. Solar wind is observed (IGY). Sticking our Head out E. Möbius UNH/SSC

4. If We Could See Our Heliospherefrom Outside … Astrosphere observed by HST ISM Wind Sticking our Head out E. Möbius UNH/SSC

5. Relevant to Exploration: Cosmic Ray Shielding Magnetic Shielding of Heliosphere Cosmic Ray Fraction Courtesy: D. McComas Sticking our Head out E. Möbius UNH/SSC

6. Plasma Flow V1 (94 AU) 84 AU Belcher

7. Interstellar neutrals LIC neutrals are not bound by magnetic fields; some enter the heliosphere. LIC H is tied to plasma via charge exchange. Slowing of plasma and neutrals in front of the heliopause creates the hydrogen wall. He H Mueller et al.

8. Pattern of the Interstellar Gas Flow Velocity Vector fromKinematics of Local Flow and Density Pattern Witte et al., Banaszkiewicz et al. Sticking our Head out E. Möbius UNH/SSC

9. LIC He from 3 Methods(Efforts of an ISSI Team) • Velocity = 26.3±0.4 km/s • Temperature = 6300±340 K • Density = 0.015±0.0015 cm-3 • LIC H Density: 0.2 ± 0.02 cm-3 • LIC magnetic field direction and strength very uncertain. Measure 1. Neutrals 2. Pickup ions 3. UV Sticking our Head out E. Möbius UNH/SSC

10. Interstellar neutrals dominate density outside ~10 AU • Pickup ions dominate thermal pressure outside 30 AU • First effects of LIC on solar wind are from these neutrals. [Mewalt]

11. Charge exchange: ion and neutral collide and ion takes an electron. H+ + H -> H + H+ New neutral H moves with plasma speed New H+ is accelerated to plasma speed and has initial thermal energy equal to the plasma energy (1 keV in solar wind): is called a pickup ion. The energy/momentum come from plasma flow, so plasma slows down.

12. Interstellar Neutral Effects on the SW • Solar Wind Slowdown • Can determine slowdown at solar maximum or when two spacecraft are at the same heliolatitude • dV/V = 6/7 Npu/Nsw

13. Solar Wind Slowdown N=0.09 cm-3 at TS (0.2 cm-3 in LIC)

14. Pickup ion energy heats the SW? Temperature profile is not adiabatic; T decreases to 25 AU, then increases. Energy comes from isotropization of pickup ion ring distributions. More heating with higher speeds About 4% of isotropization energy heats solar wind

15. Approach to the termination shock: V1 TS Foreshock V1 - no plasma data; We did not know TS location. Ions and electrons observed streaming away from Sun. (wrong direction?) After TS crossing ion intensities were steady and isotropic in sheath. The V1 TS crossing at 94 AU revealed the spatial scale of the heliosphere.

16. Shape of the Termination Shock • TS is blunt, as evidenced by streaming of foreshock beams at V1 and V2 Sticking our Head out E. Möbius UNH/SSC

17. V2 sees streaming in opposite direction than V1; consistent with the blunt shock hypothesis

18. Heliospheric Asymmetry. V1 enters TS foreshock region At 85 AU, V2 enters At 75 AU. Why? A LIC magnetic field at an angle to the flow can cause asymmetries. 84 AU 75 AU

19. Ly  Observations Have Revealed Deflected Flow of H He: original V H: deflected in outer heliosheath Asymmetric LIC Magnetic FieldDeflects the Decelerated Flow H Latitude He Longitude Lallement et al. 2005 Sticking our Head out E. Möbius UNH/SSC

20. Simulation of sheath (Opher) Tilted LIC magnetic field gives asymmetry TS and HP closer in South than North. Magnitude of asymmettry was subject of controversy B

21. Asymmetry Observed: V2 crosses the TS In Aug. 2007 at 84 AU • V2 TS Overview • Speed decrease starts 82 days, 0.7 AU before TS • Crossing clear in plasma data • Flow deflected as expected • Crossing was at 84 AU, 10 AU closer than at V1

22. BUT, TS location changes as solar wind pressure changes. So we need to predict TS motion to determine size of asymmetry. 2-D model of Chi Wang uses V2 SW pressure as input Normalized to V1 crossing Predicts TS location ~2-3 AU closer than at the V1 TS crossing Thus TS asymmetry is 7-8 AU. Modeling of asymmetry and flow in heliosheath may help pin down LIC magnetic field direction and strength.

23. Termination Shock: Expectations Neptune: Strong shock, most flow energy heats thermal ions. • Neptune BS - TS comparison • Normalize SW to values at TS • Same time scale. • Neptune BS much stronger and much more plasma heating

24. 160 190 230 X 750 Unexpected Observation 1 SW energy drops in discrete steps starting 0.7 AU upstream of TS (associated with MIRs?) 40% of SW flow energy is lost before TS Associated with MIRs; energy into particle heating?

25. 160 190 230 Energetic particles at V2 (Decker)

26. Voyager 2 Termination Shock Crossings HSH HSH SW

27. Plasma Exp. Belcher

28. TS Unexpected Observation 2 Purpose of shock is to make flow subsonic BUT, flow remains supersonic wrt thermal plasma in heliosheath. Energy must reside in pickup ions

29. Termination Shock Jumps Upstream Downstream |V| (km/s) 320.5 172 N (cm-3) 0.0012 0.0024 W (km/s) 10 50 VT (km/s) 9 38 VN (km/s) -26 -32 EW angle (deg) 1.6 14 NS angle ( deg) -5 -12

30. Termination shock with classic structure: foot, ramp, shock. (Burlaga et al.)

31. Unexpected Observation 3 Structures of TS crossings a few hours apart are very different: there appear to be two ramps in first crossing.Shock may be reforming downstream(Burlaga et al.)

32. Vn • Flow directions: as expected, flow diverts in T and -N directions • Flow in -N, +T before shock VT

33. Will V2 cross the TS again? • Dynamic pressure at 1 AU (Wind) • Decreases through 2007

34. Next Voyager milestones: the Heliopause and the Interstellar Medium Voyager Trajectories

35. Heliosheath Flow How do we know if the HP is near? 1. Flow angle changes across the HSH Mueller et al.

36. Flow turns across HSH and must be parallel to the HP at the HP. • Decker et al. show that angle is changing; time for angle to rotate 90o gives a HP thickness of 32 AU.

37. HP 2.HeliosheathPDL? • Axford-Cranfill postulate increased magnetic field at HP boundary. • Models also suggest a magnetic barrier may form. (Pogorelov et al., 2006) • Results in plasma depletion layer in model. TS PDL

38. LIC H and He • Velocity = 26.3±0.4 km/s • Temperature = 6300±340 K • Density = 0.015±0.0015 cm-3 • LIC H Density: 0.2 ± 0.02 cm-3 • LIC H+ Density : 0.06± 0.01 cm-3 Sticking our Head out E. Möbius UNH/SSC

39. Relevant to Exploration: Cosmic Ray Shielding Cosmic Ray Fraction Courtesy: D. McComas Sticking our Head out E. Möbius UNH/SSC

40. Summary • Voyager 2 crossed the TS in Aug. 2007 • Showed heliosphere is asymmetric • Shock strongly modulated by pickup ions • TS effects start 0.7 AU upstream of TS • The interstellar medium is ~10 years ahead!

41. Why Interstellar Helium?(and not Hydrogen) • Has the Highest Ionization Potential i.e. Reaches 1 AU • Can be Observed with 3 Methods: Neutrals, Pickup, Scattering of Solar UV • Second Most Abundant Species i.e. Is an Important Species in the LIC • Not Affected by the Heliospheric Interface i.e. Provides an Unbiased Account of the LIC Sticking our Head out E. Möbius UNH/SSC

42. SPEEDS ACE How do LIC neutrals effect the SW? They start the transfer of SW flow energy into heating of plasma and particles. Energy acquired by pickup ions slows down SW 1 AU (IMP 8 and ACE) and V2 speeds. V2 speeds in outer heliosphere are less than those at 1 AU. V2

43. P: 51-day running average Dynamic Pressure mnV2 Solar cycle dependence: Factor of >2 change with peak after solar maximum

44. Structure of first TS crossing is very different: there appear to be two ramps.Shock may be reforming downstream

45. Shape of the Termination Shock • It is also asymmetric: Voyager 2 sees TS signs earlier than for a symmetric Heliosphere • TS is blunt, as evidenced by streaming of foreshock beams at V1 and V2 A 3D View of the TS Is needed! V1 TS Ed Stone V2 Meraph Opher Sticking our Head out E. Möbius UNH/SSC

46. Puzzle: Where is the Anomalous Cosmic Ray Source • Ions come from maximum acceleration region along B-Field (Schwadron & McComas) • Ions come from maximum acceleration region along B-Field (Schwadron & McComas) • Acceleration in Heliosheath (Fisk & Gloeckler) • Ions come from maximum acceleration region along B-Field (Schwadron & McComas) • Acceleration in Heliosheath (Fisk & Gloeckler) • Acceleration of Pickup Ions with anisotropic PADs provides gap in spectra (Florinski) Schwadron & McComas 2006 Sticking our Head out E. Möbius UNH/SSC

47. Plasma Interaction with the LIC Plasma Density Contours Blue & Green Solar Wind Bow Shock? He He Termination Shock ISM ≈ 26 km/s H & O Charge Exchange O, H slow & hot H+, O+ Heliopause ISM Orange - Pristine ISM Red - Decelerated ISM 3D MHD Model T. Linde, Thesis Sticking our Head out E. Möbius UNH/SSC