1. Background2. Flux variation3. Polarity reversal4. Electron evolution5. Conclusions The role of...
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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
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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
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Solar cycle: photosphere
1995
Mt. Wilson magnetographs
2001
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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
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Solar cycle: corona
Yang Liu, SHINE 2006
Riley et al., 2006
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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
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Influence of CMEs on corona
Luhmann et al., 1998
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Heliospheric flux variation
• How can you add flux to the heliosphere?
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Suprathermal electrons
ab
c
d
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Interplanetary CMEs
Crooker et al., 2004
Marubashi., 1997
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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
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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
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Flux added by a single CME
Owens and Crooker, 2007
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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
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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
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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
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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
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Observable test
Owens et al, 2007 Crooker and Webb, 2006
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Crooker et al, 2008
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Transport of flux
Interchange reconnection transports open flux across CME footpoints
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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
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Rise phase
Time Owens et al, 2007
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Declining phase
Time Owens et al, 2007
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Prediction
Owens et al, 2007 Crooker and Webb, 2006
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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
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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
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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
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Owens and Crooker, 2007
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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?
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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
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Simulation with pitch-angle scattering
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
What’s going on?
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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?
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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
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Extra slides
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
The solar cycle - sunspots
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Comparison with Ulysses
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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
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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
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Suprathermal electrons for a single CME
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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
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1. Background 2. Flux variation 3. Polarity reversal 4. Electron evolution 5. Conclusions
Pich-angle scattering