# Presentation: 524720

## 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

## Our ``County‘‘ in the Cosmos: The Local Bubble

**Our ``County‘‘ in the Cosmos: The Local Bubble**

**Local Bubble and Loop I **
**are Interacting Bubbles!**

**Blasts from Past**

**ISM Cloud Flow**

**From ISSI Workshop: From the Heliosphere to the Local Bubble**

- 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.

## 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).

## If We Could See Our Heliospherefrom Outside ...

**ISM Wind**

- Astrosphere observed by HST

## Relevant to Exploration: Cosmic Ray Shielding

**Cosmic Ray **
**Fraction**

***Relevant to Exploration: Cosmic Ray Shielding***

**Courtesy*****: D. McComas***

- Magnetic Shielding of Heliosphere

**Notes:**
Add “Shields Up!”

## Plasma Flow

- V1 (94 AU)

- 84 AU

- Belcher

## 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.

- Mueller et al.

- He

- H

## Pattern of the Interstellar Gas Flow

**Velocity Vector from****Kinematics of Local Flow**
**and Density Pattern**

***Witte et al., Banaszkiewicz et al.***

## 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. 

**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. **

**LIC He from 3 Methods** **(Efforts of an ISSI Team)**

- Measure 1. Neutrals 2. Pickup ions 3. UV

## 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.

- 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.

## Solar Wind Slowdown
Can determine slowdown at solar maximum or when two spacecraft are at the same heliolatitude
dV/V = 6/7 Npu/Nsw

- Can determine slowdown at solar maximum or when two spacecraft are at the same heliolatitude
- dV/V = 6/7 Npu/Nsw

- Interstellar Neutral Effects on the SW

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

- N=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 speeds
About 4% of isotropization energy heats solar wind

- 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

## Approach to the termination shock:

- V1 - no plasma data;
- We did not know TS location.
- Ions and electrons _x0005_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.**

-  V1 TS Foreshock

## Shape of the Termination Shock

**TS is blunt, as evidenced by streaming ****of foreshock beams at V1 and V2**

**Notes:**
Add Ed Stone’s Fig. with simultaneous foreshock ions at V2!

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

## Heliospheric 

- 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

## Ly  Observations Have Revealed Deflected Flow of H

**Asymmetric LIC Magnetic Field****Deflects the Decelerated Flow**

***Lallement et al. 2005***

**He**

**H**

**Latitude**

**Longitude**

- He: original V   H: deflected in outer heliosheath

## B

- 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

## 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

- 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

- Asymmetry Observed:
-   V2 crosses the TS
- In Aug. 2007 at 84 AU

## 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.

- 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.

## 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

- 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

## 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?

- 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?

- X 750

- 160   190       230

## Energetic particles at V2 (Decker)

- 160
- 190
- 230

## Voyager 2 Termination Shock Crossings

-  SW

- HSH

- HSH

- Belcher

## 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

- Purpose of shock is to make flow subsonic
- BUT, flow remains supersonic wrt thermal plasma in heliosheath.
- Energy must reside in pickup ions

- TS

## 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

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

- (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.)

## Flow directions: as expected, flow diverts in T and -N directions
Flow in -N, +T before shock

- Flow in -N, +T before shock

- VT

- Vn

## Dynamic pressure at 1 AU (Wind)
Decreases through 2007

- Decreases through 2007

- Will V2 cross the TS again?

## Next Voyager milestones: the Heliopause and the Interstellar Medium

-                                  Voyager Trajectories

## Heliosheath Flow

- How do we know if the HP is near?
- 1. Flow angle changes across the HSH

- Mueller 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.

- Decker et al. show that angle is changing; time for angle to rotate 90o gives a HP thickness of 32 AU.

## 2. Heliosheath PDL?

- 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.

- PDL

- HP

- TS

## 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

**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**

**LIC H and He **

## Relevant to Exploration: Cosmic Ray Shielding

**Cosmic Ray **
**Fraction**

***Relevant to Exploration: Cosmic Ray Shielding***

**Courtesy*****: D. McComas***

**Notes:**
Add “Shields Up!”

## 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!

## 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***

## 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.

- 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

- ACE

-        SPEEDS

## Dynamic Pressure mnV2

Solar cycle dependence:
Factor of >2 change with peak after solar maximum

- Solar cycle dependence:
- Factor of >2 change with peak after solar maximum

- P: 51-day running average

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

## Shape of the Termination Shock

***Ed Stone***

**V1  TS**

**V2**

**TS is blunt, as evidenced by streaming ****of foreshock beams at V1 and V2**

**• ****It is also asymmetric: Voyager 2 sees TS signs earlier than for a symmetric Heliosphere**

***Meraph Opher***

**A 3D View of the TS**
**Is needed!**

**Notes:**
Add Ed Stone’s Fig. with simultaneous foreshock ions at V2!

## Puzzle: Where is the Anomalous Cosmic Ray Source

**Ions come from maximum acceleration region along B-Field (*****Schwadron & McComas*****)**

***Schwadron & McComas***** 2006**

**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*****) **

## Plasma Interaction with the LIC

*Blue & Green Solar Wind*

*Plasma Density Contours*

* **3D MHD Model  T. Linde, Thesis*

***Orange - Pristine ISM***
***Red - Decelerated ISM***

**Bow Shock?**

**Heliopause**

**ISM**

**Termination Shock**

**ISM ≈ 26 km/s**

**He**

**He**

**H & O**

**Charge Exchange**

**        ****O, H**
**slow & hot**

**H****+****, O****+**