Z.Y. Pu, J. Wang, X.G. Zhang, Q.G. Zong, S.Y. Fu, L. Xie,

1. Magnetic Reconnection at the Magnetopause: New Results from Double Star & Cluster Observations Z.Y. Pu, J. Wang, X.G. Zhang, Q.G. Zong, S.Y. Fu, L. Xie, C.J. Xiao, X.G. Wang (Peking University, China) M.W. Dunlop (Space Sciences Division, RAL, Oxford, UK) P. Escoubet (ESA/ESTEC, Noordwijk, Netherlands)

2. Outline ● Introduction ● New results of magnetic reconnection (MR) from Double Star & Cluster observations ● Summary

3. 1. Introduction

4. Magnetic Reconnection (MR) X-line- MR is a fundamental process in plasmas by which the B field topology changes and connections of particles with the field are re-arranged. MR is the major process responsible for energy transport in space plasmas.

5. MR at the Magnetopause (MP) Dungey (1961,1963) IMF southward ★ IMF northward ★ ★

6. Observations of Quasi-Steady and Transient MR at the MP ●ISEE 1/2 detected quasi-steady MR at the dayside MP(Paschmann et al. 1979) ●ISEE 1/2 & HEOS-2 identified impulsive MR (FTEs) (Russell & Elphic, 1978; Haerendel et al. 1978) X-line- quasi-steady MR Flux transfer events (FTEs)

7. Importance of MR at the MP MR at the MP is the major mechanism responsible for the solar wind energy, plasma and magnetic flux transports into the magnetosphere.

8. Cluster   TC-1 Outstanding Questions Before launching of Double Star (DS), there were ongoing debates and studies on a number of outstanding problems. DS-Cluster coordinated measurements provide a unique opportunity to study these key questions.

9. ●Anti-Parallel vs Component MR? Where MR is initiated at the dayside MP? Anti-parallel MR at high-latitude, or component MR near the sub-solar point? (e.g., Karimabadi et al., 2005; Trattner et al., 2005) Some observations were in favor of the former, others agreeed more with the latter (e.g., Petrinec et al., 2004; Retino` et al., 2005; Kim et al., 2002; Chisham et al., 1999) IMF anti-parallel MR Component MR (Gonzalez and Mozer, 1974; Sonnerup, 1974; Cowley, 1976) (Crokker,1979; Luhmann et al., 1984)

10. ●Global Configuration of the X-Line at the MP ? Various global patterns were suggested based on modeling, theoretical and observational studies X-line A Null Law and Finn (1990) Wang (1996) Fuselier et al.(2002) X-lines ● ● B Null Moore & Fox (2002)

11. ●Global View of Formation and Motion of FTEs ? Magnetosphere (MSP) Magnetosheath (MSH) (From Owen et al.) Paschmann (2002)

12. Double Star Mission Double StarTC-1was launched into an orbit of 570 km 13.4 RE, inclined at 28.5o.

13. Conjunctions between Cluster and Double Star During Jan. to April, Cluster and TC-1 have apogees in dayside solar wind, suitable for coordinated observations of MR at the dayside MP. TC1 Cluster

14. II. New results of MR from Double Star & Cluster observations ● Convincing in situ evidence for component MR near the sub-solar point (IMF By0) ●Simultaneous occurrence of anti-parallel and component MR under similar IMF conditions (IMF By0) ●Global extent & configuration of the X-Line at the MP ●Global view of formation and motion of FTEs

15. Data Processing (Pu et al. 2007) Definition of Vbackground Criterion of event selection: |Vjet| > 250 km/s Duration > 30 s Relatively steady IMF clock angle More than 1500 events were selected (Zhang, 2007) Vjet = Vobserved – Vbackground

16. Event Selection & MR Site Determination BN (Paschmann, 2002) BN VL & BN reversals Walen test Plasma mixing (Zhang, 2007) Hall effect (Oieroset et al., 2001)

17. 180o z BSP BSH y 0o Criteria for Component MR Three types for Component MR : (1) the shear angle across the local MP at the MR siteis noticeably < 180o; (2) a guide field is present inside the diffusion region; (3) The IMF Bz in the adjacent magnetosheathis positive.

18. 60 ● Convincing In Situ Evidence for Component MR Near the Sub-Solar Point MTH MSP Bz 40 By 0 ◆ Case studies: Component MR on 2004-03-26 TC-1 at ~(10.4, -2.2, -1.4) RE; IMF Bz >0; Shear angle ~60o 10 N MP 1 Shear Angle Bx 10 20 T 1 -20 Flow reversal Vz 100 Sheath Sphere Vy 0 C=0.9502 C=0.9768 Vx -100 Y=-0.74X+4.84 Y=-0.82X+2.84 (Pu et al. 2005a) 09:46:20-09:47:30UT 09:41:00-09:43:55 UT

19. ◆Statistical studies with various IMF clock angles Pu et al.(2007) Paschmann (2002) A statistical study of 176 jet events from TC-1 observations for dawnward IMF Picture from component MR hypothesis Most ⊥to jets “the direction of the flows, northerly (southerly) for TC-1 locations north (south) of the X-line, was consistent with an X-line through the sub-solar point”(Paschmann , 2008) . Most ⊥to jets Possible X-line A statistical result from TC-1 observations for duskward IMF (Zhang, 2007)

20. ●Global view of formation and motion of FTEs (06 April 2004) Z Cluster ♦ (6.1,-3.3,5.5) RE6 Y ● (3.5,-7.3,-4.6) RE6 TC-1 (Dunlop et al., 2005)

21. FTEs Observations DSP (-/+) Cluster (+/-) BN bipolar: Cluster (+/-),TC-1(-/+)

22. IMF   X-line Colling (Colling et al. 2000) dH-T FTEs are formed via component MRat the low-latitude dayside MP in pairs, and moved from the source region poleward and dawnward/duskward.

23. Cluster ◆ TC1 ★ Global view of FTEs motion and geometry(Xiao et al. 2005; Pu et al., 2005b; Wang et al2006)。 Highlights of Cluster-Double Star coordinated observation (Escoubet et a., 2005)

24. ● Simultaneous Occurrence of Anti-Parallel and Component MR under Similar IMF Conditions (IMF By0) ◆Anti-parallel MR: Operating in high-latitudes and dawn/dusk flanks Signatures:Null points; jets parallel and anti-parallel to B fields in the magnetosheath and magnetosphere, respectively ◆Component MR: Originated at the low latitudes near the sub-solar point; the MR structures can also be seen in high-latitudes

25. ◆Simultaneous Occurrence of anti-parallel and component MR on 25 Feb, 2005 Event Overview IMF duskward BL BM Cluster BN BL BM TC-1 BN IMF Clock angle 09:41 10:37 Cluster MR: anti-parallel TC-1 FTE: component MR (Dunlop et al., 2009)

26. Anti-parallel MR at high-latitude Feb 25, 2005 Null BN A BLBM B N T Null point reconstruction at high latitude MP (Dunlop et al., 2009) VLVM VN Cluster 3 (6.4,2.6,6.8) RE, GSM

27. ◆Simultaneous Occurrence of anti-parallel and component MR on 06 April, 2004 Event Overview IMF dawnward Anti-parallel MR at high-latitude BL Cluster Reversal of BN Mixing of MSH & MSP plasmas BN BL BM VM BM Reversal of VL VN VL Fast flow a Cluster ★ A N BN a T BN X line 4:40 5:20 5:00 4:00 4:20 Reversal of jet flow, parallel/anti-parallel to the B in the MSH/MSP Cluster anti-parallel MR DS FTEs (Wang et al.,2008) Component MR

28. ●Global Extent & Configuration of the X-Line at the MP ◆ 180 (±22.5)  IMF (147 events) 10 Z(RE) Z(RE) 20 10 X-line 0 0 -10 -10 Y(RE) -20 -20 Y(RE) ▲Fast jet flows are observed at almost all locations on the MP ▲Most of the jet flows are basically in Z-direction. ▲X-line is basically lying in the Y-direction.

29. ◆Dawnward IMF (468 events) Z Z(RE) 20 10 0 X-line -10 -20 Y(RE) Y -20 -10 0 10 20 ▲S-shape X-line; Similar to the previous results ( Pu et al., 2007) ▲More extended

30. ◆Duskward IMF (244 events) Z(RE) 20 Z 10 0 X-line -10 -20 Y Y(RE) -20 0 -10 10 20 ▲Well reconstructed S-shaped X line

31. ◆135° IMF (158 events) Z Z(RE) 20 10 0 X-line -10 -20 Y(RE) Y 20 0 -20 10 -10 ▲X-line elongated to the flank regions ▲Potential anti-parallel MR outflow in the flank

32. ◆-135° IMF (343 events) Z Z(RE) 20 10 0 X-line -10 -20 Y(RE) Y 0 -20 10 20 -10 ▲X-line elongated to the flank regions ▲Potential anti-parallel MR outflow in the flank

33. ● Dependence of X-line orientation on the IMF Clock Angle Rotation of X-Line orientations with different IMF clock angle : Titled angle of X-lines  IMF clock angle Component MR at Low-latitudes θ = 0.24 ( A_IMF – 180o ) + 91o (Zhang, 2007)

34. III. Summary (1) Component MR dominants at the low-latitude MP when IMF By0 and occurs (there) for both IMF Bz< 0and Bz > 0; while in high-latitudes, both anti-parallel and component MR were observed. “There is no requirement that the shear angle between the magnetic fields on the two sides of the current layer must be 180; it can be 90 or even considerably less. This is particularly relevant for the MP magnetopause, where there are always locations at which the fields are anti-parallel, regardless of the IMF direction, but nature does not appear to prefer those locations”. [From Paschmann (2008)]

35. (2) FTEs are formed via component MR at the low-latitude dayside MP in pairs, and moved from the source region poleward and dawnward/ duskward. Cluster ◆ ★ TC1

36. (3) Simultaneous occurrence of component and anti-parallel MR at the low- and high-latitude MP/ flanks respectively indicates that a large area of MP is open for IMF Bz< 0. The MR region extens across most of the dayside MP, from low-latitudes near the subsolar point, to dayside high-latitudes and the dwanside/duskside flank. Dawnward IMF Duskward IMF Moore & Fox (2002) Pu et al. (2007)

37. (4)DS & Cluster coordinated measurements support the 3D Separator MR Model that a large-scale X-line joins nulls in opposite polar regions and extends across the dayside MP. The dayside separator line displays properties of both anti-parallel and component MR, with ‘anti-parallel’ and ‘component’ being the local features of MR near nulls and separator, respectively. X-line X-line A Null Law and Finn (1990) Wang (1996) X-line ~ Separator Dorelli et al. (2007) ● ● B Null

38. Thanks! Supported by the NSFC Key Programs (No. 40731056) and China Key Research Project (No. G200000784). The authors thank PIs of Cluster and Double Star Missions for supporting the data.

39. Conjunctions between Cluster and Double Star From Jan. to Apr. 2004, 21 MP crossings within 1 h and 4within 15 min. TC1 Cluster