Thomas Zurbuchen

University of Michigan

thomasz@umich.edu

The Structure and Sources of the Solar Wind during the

Solar Cycle

Contents

• Space weather and heliospheric structure• Solar cycle

– What happens in the corona?– What happens to the heliospheric current sheet?

• Heliospheric polarity change

– What happens to the solar wind?• Identification of solar wind sources

• Conclusions

Satellite upsets: GOES

Transformer damage

Solar Cycle: Sunspot Numbers

Solar Magnetic Cycle

Propagation and Heliospheric Interaction

Geospace Interactions

Heliospheric Structure?

• Major effects of solar storms are caused by swept-up heliospheric plasma.

• Energetic particles release from the Sun follow the magnetic field in the heliosphere.

The Corona

Solar Minimum Corona

• Solar corona falls into two areas with fundamentally different magnetic topologies.

• High latitudes: coronal holes

• Low latitudes: large loops -> streamers

Solar Minimum Corona

Solar Maximum Corona

• Open magnetic field regions appear at all latitudes. In principle, still two fundamentally different magnetic topologies.

• Highly transient evolution on many time-scales.

• Current status of research: Use time-stationary MHD solution with parameterized heating.

Photospheric magnetic field

Solar Maximum Corona

Solar Minimum Corona• Overall structure

well approximated by stationary potential field model.

• Assume outside surface is conductor.

B(

r , t) r0

r0

r

n1

r0

rs

n1r

rs

n

cn

m

m0

n1

Pnm e i m t

Magnetic field in 1998.

Liewer et al., 2002.

1) Larger range of sources

2) Long magnetic connections in

Magnetic field in 2001

Linker et al., 2001.

Magnetic loops are heated

The Heliosphere

Major questions

• What is the large-scale structure of the heliosphere during the solar cycle?

• How does it relate to the magnetic structure of the corona?

• How does the heliospheric field change?

The Current Sheet (CS)

Separates magnetic polarities

Solar Minimum CS

Solar Maximum CS

Heliospheric Polarity Change

• Heliospheric field is calculated under following approximation (Riley et al, for details)

1. Measure Earth-pointing magnetic field each Carrington rotation.

2. Assume stationary condition for this time and calculate coronal magnetic field using certain approximation on heating.

3. Use fluid code to project this field into heliosphere.

Heliospheric Polarity Change

• Fundamental processes:– Magnetic flux emergence at small geometrical scale.– Differential rotation and meridional motion that pushes

and tears magnetic field and forces interactions of photospheric magnetic flux.

– Reconnection between magnetic field lines, open and closed.

• Neither of these processes are well understood and currently modeled!

Surface Motions

Images from Stanford Observatory

Reconnection effects

Loop -loop

Loop –open field line

A physical model: Fisk and Schwadron, 2001; Zurbuchen et al., 2004

• Realize that there are open and closed field lines that have their constraints.

• Solar wind comes from closedstructures, released by reconnection.

• Open heliospheric magnetic field lines:

rSSrSr BuB

t

B

2

“diffusive” motionsdue to reconnections

convective motions- differential rotation- meridional flow

Solar minimum,high latitudes rSSrS

r BuBt

B

2

Solar wind sources

• Common assumption by modelers: – All solar wind comes from coronal holes.– Coronal heating differences produce fast and

slow solar wind.

• Composition data:– There are two fundamentally different sources

of solar wind that result in fast and slow flows.– These sources directly relate to the

reconnection processes just described!

Composition

• Solar wind elemental composition from Ulysses.

• Both, solar maximum and solar minimum solar wind consist of coronal hole associated and streamer associated wind.

Zurbuchen et al., 2002.

Slow solar windSources

• Compositional patterns in coronal loops of 100 Mm are observed to be like slow solar wind.

• These loops are likely source of slow solar wind.

• Plasma must be released through reconnection.

Important questions

• Fundamental processes:– Magnetic flux emergence at small geometrical scale.– Differential rotation and meridional motion that pushes and

tears magnetic field and forces interactions of photospheric magnetic flux.

– Reconnection between magnetic field lines, open and closed.

• What are the reasons for these processes inside the Sun?

• What is the physics of magnetic reconnection?• How does it relate to solar wind and particle

acceleration?

Solar Minimum Sources

Solar Maximum Sources

Summary

• Understanding and predicting space weather and its engineering consequences has become very important.

• This will require major progress in fundamental knowledge of solar processes currently not well known.

• We are at the transition of fundamental research and applications of interest for industry and the Nations economy.

That’s it…

Space Weather Damages

• Upsets and failures of space systems due to space radiation

• Satellite life-time issues due to enhanced atmospheric drag

• Powergrid disruptions

• Negative effects on communications systems and GPS.