1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

The role of coronal mass ejections in the solar cycle evolution of the

heliospheric magnetic field

M.J. Owens, N.U. Crooker, N.A. Schwadron, H.E. Spence and W.J. Hughes

Center for space physicsBoston University

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Overview

1. Background

2. Heliospheric flux variation

3. Heliospheric polarity reversal

4. Suprathermal electrons

5. Conclusions

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Solar cycle: photosphere

1995

Mt. Wilson magnetographs

2001

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Solar cycle: Heliosphere

Jones et al., 2003e.g. Richardson et al., 2002

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Solar cycle: corona

Yang Liu, SHINE 2006

Riley et al., 2006

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

How does the coronal field evolve?

• Wang & Sheeley: Emerging loops bring about field reversal by destruction of existing open flux– Series of PFSS solutions

• Fisk & Schwadron: Open flux is conserved, but reconfigured by reconnection

• B.C. Low: Magnetic helicity conservation means potential state cannot be reached by reconnection alone– CMEs required to shed the helicity

– CMEs bodily remove flux to allow field reversal

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Influence of CMEs on corona

Luhmann et al., 1998

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Heliospheric flux variation

• How can you add flux to the heliosphere?

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Suprathermal electrons

ab

c

d

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Interplanetary CMEs

Crooker et al., 2004

Marubashi., 1997

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

ICMEs contain closed fields

Riley et al., 2004

1 AU: Shodhan et al., 2002

5 AU: Crooker et al., 2002

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Flux added by ICMEs must be removed

No “flux catastrophe” – McComas et al, 1992– Equivalent fields must open

Two possibilities:– Disconnect open fields

– Open CME closed loops via interchange reconnection (Crooker et al., 2002)

a

b

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Flux added by a single CME

Owens and Crooker, 2007

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Timescale for flux opening

• Disconnection and interchange reconnection add/remove flux at same rate if rate of reconnection is the same

• Assume exponential decay to flux from a single CME added to heliosphere

t – time since launchφ – flux contained in CMED – fraction of flux which opens at launchλ – decay constant

Interchange

Disconnection2

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Heliospheric flux budget

Assume a constant CME rate:

Equate open flux at min/max (i.e., assume variation in |B| is entirely due to ICMEs)

T1/2 ~ 40 days

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

LASCO-driven simulation

• LASCO CMEs have been catalogued.

Use LASCO CME times to drive simulation.

• At each time-step, insert new CMEs and decay flux from existing ICMEs.

• Observed variability in |B| can be very well matched

Owens and Crooker, 2006

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Suprathermal electrons• Method of reconnection important for

heliospheric field evolution

• Simple picture:– Interchange: no EDs, decay in CSE

– Disconnection: EDs, no decay in CSE

a

b

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Observable test

Owens et al, 2007 Crooker and Webb, 2006

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Crooker et al, 2008

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Transport of flux

Interchange reconnection transports open flux across CME footpoints

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

CME footpoints•Polarity of CME footpoints.

– Magnetic cloud observations

Bothmer and Schwenn, 1998

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Rise phase

Time Owens et al, 2007

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Declining phase

Time Owens et al, 2007

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Prediction

Owens et al, 2007 Crooker and Webb, 2006

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

• Number of CMEs required to reverse polarity:

Is there sufficient flux?

• Timescale for such a reversal

d > 5o

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Suprathermal electrons• Method of reconnection important for

heliospheric field evolution

• Simple picture:– Interchange: no EDs, decay in CSE

– Disconnection: EDs, no decay in CSE

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Suprathermal electron

scattering

Fra

ctio

n o

f to

tal e

lect

ron

den

sity 1.00

0.10

0.01

0.3 0.6 1 2Heliocentric distance (AU)

corehalo

strahl

Maksimovic et al., 2005

Hammond et al., 1996

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Owens and Crooker, 2007

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

How long do closed loops retain the CSE signature?

• Scattering process is still a topic of research

• Empirically match observed scattering rate– Can a constant scattering rate reproduce the

switch with distance of focusing to scattering?

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Numerical simulation

• Parker Spiral magnetic field

• Halo electrons move into weaker fields

• Magnetic moment

– μ = VPERP2/B

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Simulation with pitch-angle scattering

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

What’s going on?

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Next steps..

• Generalise electron model to closed loops

• Determine length of loop when CSE signature is removed– If it is large, we can we discount reconnection

because of too few CSE signatures?

– What are the implications for the heliospheric flux budget?

– Is the scattering rate in magnetic clouds the same as in the ambient solar wind?

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Summary

• The solar cycle manifests itself in the heliosphere as:– A doubling of the heliospheric flux

– A reversal/rotation of the heliospheric current sheet

• Coronal mass ejections can explain these observations by:– Temporarily adding closed flux to the heliosphere

– Transporting open flux across CME footpoints by interchange reconnection close to the Sun

• The distance at which closed loops lose their identity is important for the heliospheric flux budget

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Extra slides

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

The solar cycle - sunspots

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Comparison with Ulysses

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Simulation – sine-fit

Use simple sine-wave fit to observed CME frequency

Owens and Crooker, 2006

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Heliospheric flux

Solar cycle variation– Approximately doubles

over solar cycle

– Returns to same value each minimum

Richardson et al [2002]: Variation is carried by ambient solar wind, not associated with ICME signatures.

Richardson et al., 2002

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Suprathermal electrons for a single CME

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

LASCO-driven simulation

• At each time-step, insert new CMEs and decay flux from existing ICMEs.

• Both interchange and disconnection can explain CSE/EDs observed

Different scattering distance

1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions

Pich-angle scattering