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

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The Voyagers at the Edge of the Heliospheric Bubble John 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.
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The Voyagers at the Edge of the Heliospheric BubbleJohn D. Richardson (MIT)With thanks to the Voyager team and E. MobiusOutlineOur place in the GalaxyThe Local Interstellar Cloud (LIC)The Solar Wind - LIC interactionThe Voyagers at the Termination ShockThe FutureOur ``County‘‘ in the Cosmos: The Local BubbleLocal 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 PastISM Cloud FlowFrom ISSI Workshop: From the Heliosphere to the Local BubbleSticking our Head out E. Möbius UNH/SSCApropos: Sticking Our Head OutHeliosphere: pressure balance between solar wind and local interstellar medium.Magnetized plasmas cannot mix. Boundary is the HeliopauseShocks form in both flows, plasma moves downstream. Solar wind is observed (IGY).Sticking our Head out E. Möbius UNH/SSCIf We Could See Our Heliospherefrom Outside …Astrosphere observed by HSTISM WindSticking our Head out E. Möbius UNH/SSCRelevant to Exploration: Cosmic Ray ShieldingMagnetic Shielding of HeliosphereCosmic Ray FractionCourtesy: D. McComasSticking our Head out E. Möbius UNH/SSCPlasma FlowV1 (94 AU)84 AUBelcherInterstellar neutralsLIC 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.HeHMueller et al.Pattern of the Interstellar Gas FlowVelocity Vector fromKinematics of Local Flowand Density PatternWitte et al., Banaszkiewicz et al.Sticking our Head out E. Möbius UNH/SSCLIC 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. UVSticking our Head out E. Möbius UNH/SSCInterstellar 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]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.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
  • Solar Wind SlowdownN=0.09 cm-3 at TS(0.2 cm-3 in LIC)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 speedsAbout 4% of isotropization energy heats solar windApproach to the termination shock: V1 TS ForeshockV1 - 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.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/SSCV2 sees streaming in opposite direction than V1; consistent with the blunt shock hypothesisHeliospheric Asymmetry.V1 enters TS foreshock regionAt 85 AU, V2 entersAt 75 AU.Why? A LIC magnetic field at an angle to the flow can cause asymmetries.84 AU75 AULy  Observations Have Revealed Deflected Flow of HHe: original V H: deflected in outer heliosheathAsymmetric LIC Magnetic FieldDeflects the Decelerated FlowHLatitudeHeLongitudeLallement et al. 2005Sticking our Head out E. Möbius UNH/SSCSimulation of sheath (Opher)Tilted LIC magnetic field gives asymmetryTS and HP closer in South than North.Magnitude of asymmettry was subject of controversyBAsymmetry Observed: V2 crosses the TSIn 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
  • 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 inputNormalized to V1 crossingPredicts TS location ~2-3 AU closer than at the V1 TS crossingThus TS asymmetry is 7-8 AU.Modeling of asymmetry and flow in heliosheath may help pin down LIC magnetic field direction and strength.Termination Shock: ExpectationsNeptune: 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
  • 160 190 230X 750UnexpectedObservation 1SW energy drops in discrete steps starting 0.7 AU upstream of TS (associated with MIRs?)40% of SW flow energy is lost before TSAssociated with MIRs; energy into particle heating?160190230Energetic particles at V2 (Decker)Voyager 2 Termination Shock CrossingsHSHHSH SWPlasma Exp. BelcherTSUnexpected Observation 2Purpose of shock is to make flow subsonicBUT, flow remains supersonic wrt thermal plasma in heliosheath.Energy must reside in pickup ionsTermination Shock Jumps Upstream Downstream |V| (km/s) 320.5 172N (cm-3) 0.0012 0.0024W (km/s) 10 50VT (km/s) 9 38VN (km/s) -26 -32EW angle (deg) 1.6 14NS angle ( deg) -5 -12 Termination shock with classic structure: foot, ramp, shock.(Burlaga et al.)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.)Vn
  • Flow directions: as expected, flow diverts in T and -N directions
  • Flow in -N, +T before shock
  • VTWill V2 cross the TS again?
  • Dynamic pressure at 1 AU (Wind)
  • Decreases through 2007
  • Next Voyager milestones: the Heliopause and the Interstellar Medium Voyager TrajectoriesHeliosheath FlowHow do we know if the HP is near?1. Flow angle changes across the HSHMueller et al.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.
  • HP2.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.
  • TSPDLLIC 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/SSCRelevant to Exploration: Cosmic Ray ShieldingCosmic Ray FractionCourtesy: D. McComasSticking our Head out E. Möbius UNH/SSCSummary
  • 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!
  • 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 SPEEDSACEHow 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 SW1 AU (IMP 8 and ACE) and V2 speeds.V2 speeds in outer heliosphere are less than those at 1 AU.V2P: 51-day running averageDynamic Pressure mnV2Solar cycle dependence:Factor of >2 change with peak after solar maximumStructure of first TS crossing is very different: there appear to be two ramps.Shock may be reforming downstreamShape 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 TSIs needed!V1 TSEd StoneV2Meraph OpherSticking our Head out E. Möbius UNH/SSCPuzzle: 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 2006Sticking our Head out E. Möbius UNH/SSCPlasma Interaction with the LICPlasma Density ContoursBlue & Green Solar WindBow Shock?HeHeTermination ShockISM ≈ 26 km/sH & OCharge Exchange O, Hslow & hotH+, O+HeliopauseISMOrange - Pristine ISMRed - Decelerated ISM3D MHD Model T. Linde, ThesisSticking our Head out E. Möbius UNH/SSC
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