Acceleration of ACRs at a Blunt Termination Shock:

2-D Simulations

J. Kόta University of Arizona

Tucson, AZ 85721-0092, USA Thanks: J.R. Jokipii, J. Giacalone

SHINE Nova Scotia, August 2009

kota@lpl.arizona.edu

● V-1

● V-2

● Are Anomalous Cosmic Rays (ACRs) indeed accelerated at the solar wind termination shock (TS) ?

Likely yes but

● Bluntness of TS counts ● Topology between shock & field Lines counts (cannot be modeled in 1 D)● Model still qualitative

Do not consider other important effects, like dynamical variations

Difference between 1 & 2 D Shocks

Voyager-1 fooled us with (1) “anti-sunward” precursor anisotropies

Solution: field line intersects the TS multiple times.

V-2

V-1

Multiple intersection explains precursor anisotropies and ….

Displacement of the ‘nose’ helps

Voyagers fooled us with(2) spectra did not unfold at crossing the TS

Solution: field lines .….?

ACR fluxes continuedto increase into theHeliosheath

● Temporal variaton(Florinski Zank,2006)

● Magnetic topology(McComas & Schwadron,Kόta & Jokipii)

● Combination of the two?

Can be a direct result of 2D topologyCould have been foreseen (Kόta & Jokipii, 2004)

Short time foracceleration

McComas and Schwadron (2006)Blunt ShockInjection & Acceleration at Flanks

Kóta and Jokipii, 2004

2D simulation of Blunt TS (offset circle)- no latitudinal motion -

This Simulation: Shock & Injection stronger at nose, weaker toward tail

More TSP at nose (injection profile) Less ACRs at nose (global feature)

2 D simulation (offset circle) cont’d

Nose-tail asymmetryControlled by κ┴

Simulated spectrum unfolds gradually

ACR flux continues to increase beyond TS

Tracing back ACRs

• Solve Parker’s equation “backward”, with the solar wind blowing inward. What we obtain is the “chance” function which is to be convolved with injection.

• Inward wind advects trajectories back to the TS, where pseudo-particles cool-down to injection energy.

• Ideally suited for GCRs (all trajectories leave sooner ot later the heliosphere. More cumbersome for ACRs

”Backward tracing” starting w5 MeV ACR 10 AU off the TS

Starting energy 5 MeV Cooled down to 100 keV

5 MeV

Chance to become 5 MeV ACR10AU off the shock

Nose (V-1) Flank 60 West

acceleration cooling

Real numbers

Age distibution

Nose&60E

Reverse method w larger κ Forward method w smaller κ

ACRs are `older’ deeper in the HS

Implications:

o ACRs are best accelerated if injected at front (more time for acceleration)

o Birthplace at Nose: Likely most of all ACRs (even those in tail) were injected at front.

o Nursery toward Flanks: TSP seen by Voyagers is the seed population of MeV ACRs. TSPs moving toward flanks during further acceleration.

One word on Precursor Events:

Possible scenarios for Voyager

o Scenario (M* ) is more efficient to accelerate energetic particles

o Voyager precursor events may have been associated with configuration M*

Less efficient- More efficient

M*

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Summary: ●V-1

● Magnetic field lines cross the blunt TS multiple times. This explains upstream anisotropies and :

● Two-population spectrum: ACRs start as TSPs at the nose and move toward the flanks during acceleration. Appear still modulated at the TS, and continue to increase into the heliosheath.

● 2-D Shock differs from 1-D shock (topology)

● Dependence on parameters (κ) still need to be explored .

●V-2

Global features are insensitive injection profile

• The distribution & spectrum of MeV ACRs turn out largely insensitive to the injection-profile along the shock.

• Lower ACR intensity is obtained at the nose even if - injection rate and/or shock ratio is higher at nose Reason: unfavourable topology (natural cold spot)

• To trace the history of ACRs we perform a “backward“ simulation. The solar wind is reversed and a pseudo ‘testparticle’ is released from the point of observation. What we obtain is the Green-function or chance of injected particle to become ACR

Illustrative example of 2-D shock

- field/shock angle alternates -

Global structure along shock frontorganized by magnetic field

Along shock front

Distance from shock

coldhot

“nose” “tail”

Motivation: where is the source?is history repeating itself ?

Do we need a new paradigm ? Likely not

ACR fluxes continued to increase beyond TS

V. Hess 1912

Voyager-1 December 2004 Similar result from V-2 (2007)

Source outside

Shock

Global structure of Heliosphere

GCR

ACR SEP

VLISM: partially ionizedH,He0.1/cc μG B ?