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

1. 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 physics Boston University

2. Overview • Background • Heliospheric flux variation • Heliospheric polarity reversal • Suprathermal electrons • Conclusions

3. Solar cycle: photosphere 1995 2001 Mt. Wilson magnetographs

4. Jones et al., 2003 e.g. Richardson et al., 2002 Solar cycle: Heliosphere

5. Solar cycle: corona Riley et al., 2006 Yang Liu, SHINE 2006

6. 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

7. Influence of CMEs on corona Luhmann et al., 1998

8. Heliospheric flux variation • How can you add flux to the heliosphere?

9. Suprathermal electrons c a b d

10. Interplanetary CMEs Marubashi., 1997 Crooker et al., 2004

11. ICMEs contain closed fields 1 AU: Shodhan et al., 2002 Riley et al., 2004 5 AU: Crooker et al., 2002

12. 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

13. Flux added by a single CME Owens and Crooker, 2007

14. 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 launch φ – flux contained in CME D – fraction of flux which opens at launch λ – decay constant Interchange 2 Disconnection

15. 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

16. 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

17. 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

18. Observable test Owens et al, 2007 Crooker and Webb, 2006

19. Crooker et al, 2008

20. Transport of flux Interchange reconnection transports open flux across CME footpoints

21. CME footpoints • Polarity of CME footpoints. • Magnetic cloud observations Bothmer and Schwenn, 1998

22. Rise phase Owens et al, 2007 Time

23. Declining phase Owens et al, 2007 Time

24. Prediction Owens et al, 2007 Crooker and Webb, 2006

25. Is there sufficient flux? • Number of CMEs required to reverse polarity: • Timescale for such a reversal d > 5o

26. Suprathermal electrons • Method of reconnection important for heliospheric field evolution • Simple picture: • Interchange: no EDs, decay in CSE • Disconnection: EDs, no decay in CSE

27. Fraction of total electron density 1.00 0.10 0.01 core halo strahl 0.3 0.6 1 2Heliocentric distance (AU) Suprathermal electron scattering Maksimovic et al., 2005 Hammond et al., 1996

28. Owens and Crooker, 2007

29. 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?

30. Numerical simulation • Parker Spiral magnetic field • Halo electrons move into weaker fields • Magnetic moment • μ = VPERP2/B

31. Simulation with pitch-angle scattering

32. What’s going on?

33. 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?

34. 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

35. Extra slides

37. The solar cycle - sunspots

38. Comparison with Ulysses

39. Simulation – sine-fit Use simple sine-wave fit to observed CME frequency Owens and Crooker, 2006

40. 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

41. Suprathermal electrons for a single CME

42. 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

43. Pich-angle scattering