Evolution of Magnetic Fields from the Sun’s Surface to the Heliopause of one Solar Cycle

Nathan Schwadron, Boston University

Outline

• Heliosphere’s Hale Cycle

• Possibilities for Reversing the Heliosphere’s

Magnetic Field

• The Role of CMEs in Field Evolution

• Observational Support

• Next Steps

• Conclusions

The Helisphere’s Hale Cycle

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Evolving Magnetic

Flux

Wang et al., 2000

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Roughly factor x2 variationin Open Magnetic Flux

QuestionsHow does the global

Field Reverse?

What causes variation in Open field strength?

Interchange reconnection

(Media Diffusion)

Interchange reconnection conserves

open magnetic fluxProvides a means for

reversal of open magnetic flux without

changing the net quantity of flux

Fisk and Schwadron, 2001

The Role of CMEs

CMEs move flux via interchange reconnection

CMEs temporarily add closed magnetic flux,

increasing |B|, particularly during solar

max

Temporary Flux Addition by

CMEs

Black line shows the observed value of |B| at 1 AU from ACE/Wind

Red line shows simulated value of |B| at 1 AU using LASCO observed

CME rates

CME half-life 50 days

Model comparison validates open flux conservation during

CME ejections

CMEs and the Hale Cycle

• CMEs generally have an orientation in agreement in with Hale’s law (Bothmer and Schwenn, Ann. Geophys., 1998)

• Is the flux opened by Interchange Reconnection sufficient for field reversal?

‣ Fopen = Open Flux ~1015 Wb

‣ FCME=CME flux ~1012 Wb

‣ freq = CME frequency ~ 3/day

‣ d = CME footpoint separation

‣ Number of Needed CMEs,

N =(Fopen/FCME)x(180o/d)

‣ Time required for field reversal,

T = N/freq ≈ 11 years

‣ Solve for footpoint separation

d > 5o

Owens et al., 2006

Owens et al., 2006

Confirmation from Suprathermal Electron

Observations• Does Interchange

Reconnection open fields primarily in one or both legs of the CME

• Answer: One

85%

15%

Crooker et al., 2006

Disconnection?

• Disconnection across the helmet streamer is another process that leads to the reduction of open flux

• This may be balanced by the addition of flux by CMEs

• It is difficult to differentiate between disconnection and the effects of large-scale interchange reconnection (flux addition, then I.C.)

Long-Term Evolution

• Evidence of Open Flux Conservation over Large Timescales (Svalgaard and Cliver, 2007)

Outer Heliosphere Implications

New Outer Heliosphere Current Layer

Next Steps• Is the Open Magnetic Flux of the Heliosphere

Conserved?‣ Recent Ulysses results may suggest disagreement with Source

surface models after the field reversal?

‣ SHINE session: Svalgaard, Murphy, Arge invited speakers

• Essential at this stage that energetic particle and magnetic field models take into account flux conservation

‣ The Earth-Moon-Mars Radiation Environment Model is designed to couple EP models w/evolving MHD results of the global solar wind

‣ Approach is to develop a mesh in which node lines follow magnetic field lines

‣ Starightforward to use the code framework to understand implications of footpoint motions for the magnetic field over the solar cycle.

Conclusions

• Coronal Mass Ejections may control the heliospheric Hale Cycle

‣ Changes in |B| through temporary buildup of open flux‣ Movement of open flux through Interchange Reconnection‣ Reversal of the Heliospheric Magnetic Field

• Next Steps‣ Is the Open Magnetic Flux of the Heliosphere Conserved?

- On what timescales and why - fundamental to solar dynamo

‣ Investigate detailed implications for energetic particles (EMMREM) and outer heliosphere configuration

- Utilize EPs to probe global structure (requires coupling between EP models and MHD/field models)

Temporary Flux Addition due to CMES

Observed

CME freq

Modeled

Owens and Crooker, JGR, 2006

•CME flux half-life ~40-55 days

Solar Butterfly Diagram

Hathaway, 2003

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