Magnetic Reconnection at the Magnetopause New Results from

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)

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

1. Introduction

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 rearranged. MR is the major process responsible for energy transport in space plasmas.

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

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)

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

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. Cluster TC-1

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 (Crokker, 1979; Luhmann et al. , 1984) Component MR (Gonzalez and Mozer, 1974; Sonnerup, 1974; Cowley, 1976)

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

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

Double Star Mission Double Star TC-1 was Double Star launched into an orbit of 570 km 13. 4 RE, o. inclined at 28. 5

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. TC 1 Cluster

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 ●

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

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

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

Convincing In Situ Evidence for Component MR Near the Sub-Solar Point ● ◆ Case studies: Component MR on 2004 -03 -26 TC-1 at ~(10. 4, -2. 2, -1. 4) RE; IMF Bz >0; Shear angle ~60 o 60 40 20 0 -20 10 1 100 MTH By Bz MP Shear Angle MSP Bx Flow reversal N T Vy Vz Vx Sheath C=0. 9502 Sphere C=0. 9768 0 Y=-0. 74 X+4. 84 -100 (Pu et al. 2005 a) Y=-0. 82 X+2. 84 09: 46: 20 -09: 47: 30 UT 09: 41: 00 -09: 43: 55 UT

◆ Statistical studies with various IMF clock angles Pu et al. (2007) Paschmann (2002) Picture from component MR hypothesis Most ⊥to jets Possible X-line A statistical result from TC-1 observations for duskward IMF (Zhang, 2007) Most ⊥to jets A statistical study of 176 jet events from TC-1 observations for dawnward IMF “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)

● Global view of formation and motion of FTEs (06 April 2004) Z Cluster ♦ (6. 1, -3. 3, 5. 5) RE 6 ● TC-1 (3. 5, -7. 3, -4. 6) RE 6 (Dunlop et al. , 2005) Y

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

IMF Colling (Colling et X-line al. 2000) d. H-T 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 ◆ ★ TC 1 Global view of FTEs motion and geometry (Xiao et al. 2005; Pu et al. , 2005 b; Wang et al 2006)。 Highlights of Cluster-Double Star coordinated observation (Escoubet et a. , 2005)

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

◆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 09: 41 IMF Clock angle 10: 37 TC-1 FTE: component MR Cluster MR: anti-parallel (Dunlop et al. , 2009)

Anti-parallel MR at high-latitude Feb 25, 2005 BN Null BL BM A N T B VL VM VN Cluster 3 (6. 4, 2. 6, 6. 8) RE, GSM Null point reconstruction at high latitude MP (Dunlop et al. , 2009)

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

● Global Extent & Configuration of the X-Line at the MP ◆ 180 (± 22. 5) IMF (147 events) 20 10 Z(R ) E Z(RE) 10 0 0 -10 -20 ▲ ▲ ▲ X-line Y(RE) -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.

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

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

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

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

Dependence of X-line orientation on the IMF Clock Angle ● : Titled angle of X-lines Rotation of X-Line orientations with different IMF clock angle θ = 0. 24 ( A_IMF – 180 o Component MR at Low)+ latitudes (Zhang, 2007) 91 o

III. Summary (1) Component MR dominants at the lowlatitude MP when IMF By 0 and occurs (there) for both IMF Bz< 0 and Bz > 0; while in highlatitudes, 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 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)]

(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 ◆ ★ TC 1

(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 highlatitudes and the dwanside/duskside flank. Dawnward IMF Duskward IMF Pu et al. (2007) Moore & Fox (2002)

(4) DS & Cluster coordinated measurements support the 3 D Separator MR Model that a large-scale Xline 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. Law and Finn (1990) Wang (1996) X-line A Null ● ● B Null X-line ~ Separator Dorelli et al. (2007)

Thanks! Supported by the NSFC Key Programs (No. 40731056 ) and China Key Research Project (No. G 200000784). The authors thank PIs of Cluster and Double Star Missions for supporting the data.

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