m.j. owens, n.u. crooker, n.a. schwadron, h.e. spence and w.j. hughes

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


Overview

1. Background

2. Heliospheric flux variation

3. Heliospheric polarity reversal

4. Suprathermal electrons

5. Conclusions


Solar cycle: photosphere

1995

Mt. Wilson magnetographs

2001


Solar cycle: Heliosphere

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


Solar cycle: corona

Yang Liu, SHINE 2006

Riley et al., 2006


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


Influence of CMEs on corona

Luhmann et al., 1998


Heliospheric flux variation

• How can you add flux to the heliosphere?


Suprathermal electrons

ab

c

d


Interplanetary CMEs

Crooker et al., 2004

Marubashi., 1997


ICMEs contain closed fields

Riley et al., 2004

1 AU: Shodhan et al., 2002

5 AU: Crooker et al., 2002


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


Flux added by a single CME

Owens and Crooker, 2007


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


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


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



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


Observable test

Owens et al, 2007 Crooker and Webb, 2006


Crooker et al, 2008


Transport of flux

Interchange reconnection transports open flux across CME footpoints


CME footpoints•Polarity of CME footpoints.

– Magnetic cloud observations

Bothmer and Schwenn, 1998


Rise phase

Time Owens et al, 2007


Declining phase

Time Owens et al, 2007


Prediction

Owens et al, 2007 Crooker and Webb, 2006


• Number of CMEs required to reverse polarity:

Is there sufficient flux?

• Timescale for such a reversal

d > 5o


Suprathermal electrons• Method of reconnection important for

heliospheric field evolution

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

– Disconnection: EDs, no decay in CSE


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


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?


Numerical simulation

• Parker Spiral magnetic field

• Halo electrons move into weaker fields

• Magnetic moment

– μ = VPERP2/B


Simulation with pitch-angle scattering


What’s going on?


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?


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


Extra slides


The solar cycle - sunspots


Comparison with Ulysses


Simulation – sine-fit

Use simple sine-wave fit to observed CME frequency



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


Suprathermal electrons for a single CME


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


Pich-angle scattering

m.j. owens, n.u. crooker, n.a. schwadron, h.e. spence and w.j. hughes

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