Licentiate thesis by Alessandro Retin ò Department of Astronomy and Space Physics

1. Magnetic reconnection at the Earth’s magnetopause:CLUSTER spacecraft observations at different scales Licentiate thesis by Alessandro Retinò Department of Astronomy and Space Physics Uppsala University

2. Motivation • Magnetic reconnection - an universal mechanism to transfer mass, momentum and energy across boundary layers in plasmas • Earth’s magnetosphere - the best laboratory to study in-situ magnetic reconnection • Magnetic reconnection affects for long time large volumes in space (large scales) but it is fast initiated in small diffusion regions (small scales) • Cluster spacecraft first magnetospheric mission with multi-point measurements

3. Outline • The solar wind - Earth interaction • Magnetic reconnection: basics • Reconnection at the Earth’s magnetopause • The Cluster mission • My contribute • Paper I - large scales • Paper II - small scales

4. The Earth’s magnetosphere

5. Magnetic reconnection in the magnetosphere DR DR Day (2001) IMF BZ<0

6. Magnetic reconnectionLaboratory, Sun, Astrophysics Solar flares Hall effect during magnetic reconnection in a laboratory plasma Ren (2005) Accretion disks & astrophysical jets

7. Magnetic reconnectionBasic ideas It is a local process (initiated at small scales) that: • changes the magnetic connectivity of plasma elements • changes the global magnetic field topology • converts energy from magnetic fields to charged particles

8. Magnetic reconnectionDefinitions 2D steady-state (good approximation) 3D DR Priest & Forbes (2000) • X-point where two separatrices meet • E along the X-line • Change in magnetic connectivity (breaking frozen-in condition) • Plasma flow across separatrices • General Magnetic Reconnection: “breakdown of magnetic connection due to a localized non-idealness“. Necessary and sufficient condition:

9. Magnetic reconnectionOur "definition" • Change in magnetic topology: • Bn at the current sheet • E||≠0 in the diffusion region • Plasma transport across the current sheet : • plasma distribution functions • plasma composition • Energy conversion from B to the plasma: • plasma acceleration in the current sheet/boundary layer • plasma heating • Particle acceleration : • strong & localized E at boundaries • strong & localized currents at boundaries

10. Magnetic reconnectionTheoretical models inflow outflow Parker (1957), Sweet (1958) Petscheck (1964) • Reconnection rate = u0/uA0 = Bn/B0 • Alfvenic outflow: ue=uA0 • Energy conversion: WB = ½ WK + ½ WT • Reconnection rate too small! • Faster reconnection • Smaller diffusion region • Particle accelerate at shocks (separatrices)

11. Evidence at large scales Fluid Walén test (tangential balance stress): • Direct method: Δut= ± ΔBt /(μ0ρ)1/2 • deHoffmann-Teller (HT) frame: u-uHT= ± uA Treumann & Baumjohann (1996)

12. Evidence at large scales Kinetic Magnetosheath boundary layer (MSBL): • Transmitted magnetospheric • Incident magnetosheath • Reflected magnetosheath Magnetospheric boundary layer (BL): • Transmitted magnetosheath with |u||i|>|uHT| • Incident magnetospheric • Reflected magnetospheric Cowley (1995)

13. Evidence at small scales • Ion diffusion region • Electron diffusion region • Magnetic separatrices Mozer (2002)

14. The CLUSTER mission • Europen Space Agency (ESA) cornerstone • 4 identical spacecraft at variable separation • For first time possible to distinguish between spatial and temporal structures • IRF-U main responsability EFW instrument (Electric fields and waves)

15. rec. rate cont. & unsteady cont. & steady intermittent Paper IMotivation • Continuity in time Southward IMF = YES Northward IMF = ? • Antiparallel vs component remote,simulations = component in-situ = ? time Reconnection is continuous if the reconnection rate ≠0

16. duskside MP • southern emisphere • tailward of the cusp • northward IMF Paper IOverview IMF • orbit + SC configuration ideal • Large # MP crossings over ~4h • SC/3 local monitor in MSH

17. ● sunward ▲ tailward Paper IReconnection continuous for northward IMF • in-situ at MP • ~4 hours continuous • northward IMF, By variable

18. jet reversal 100° shear jet reversal 160°shear Paper IComponent reconnection • measure magnetic shear at X-line • jet reversals ⇒ close to X-line • magnetic shear at X-line: ~180º antiparallel <180º component

19. Summary Paper I Large scales • Magnetic reconnection continuous for hours under northward IMF • In-situ evidence of component reconnection

20. Paper IIMotivation • Microphysics of reconnection • Few detailed observations • Separatrices close to the X-line

21. rotational discontinuity separatrix region tailward jet Paper IIOverview SC/3 only (small scales) • high resolution observations close to the X-line • separatrix region between magnetic separatrix and reconnection jet • distance from X-line < 60λsh,i ~ 3000 km (from comparison with simulations)

22. Paper IIThe separatrix region (SR) • magnetic separatrix identified as boundary in waves • SR ~ 5 λsh,i wide • inside SR few subregions ~ λsh,i wide • ESW at the boundary with jet but not inside the separatrix region • μ-FTE 3 2 1 bulge ESW jet magnetic separatrix

23. 1 3 2 Paper IIWave-particle interaction inside the SR • wave-particle interaction mainly from simulations • in observations need simultaneous high-time resolution wave and particle measurements • here best observations in the separatrix region (instrument limit resolution) • subregions different properties • good agreement with simulations 0° away from X-line 180 ° towards X-line

24. Paper IIA sketch of the separatrix region • SR and its subregions spatial structures • bulge (μ-FTE) temporal structure

25. Summary Paper II Small scales • A separatrix region several ion lenghts wide exists on the magnetospheric side of the magnetopause ~ 60 ion lengths away from the X-line. • This region contains a few subregions each about oneion lenght wide. • These subregions are highly structured down to the Debye scale. • Comparison with numerical simulations indicates good agreement although some features observed at small scales are not resolved.

26. Future work • Reconnection continuous at large temporal scales.Also true at short temporal scales ~ 1 s? Or intermittent instead? μ-FTEs maybe important. • Component or antiparallel reconnection at the magnetopause? More in-situ measurements of magnetic shear. • The microphysics of magnetic reconnection: • X-line crossings • Evidence of ion and electron diffusion regions • Separatrix region crossings away from X-line • Energy conversion • Transport across • The relationship between the diffusion region and the separatrix region. Is the separatrix region a direct extension of the diffusion region far away from the X-line? • Comparison with reconnection magnetotail observations. Different boundary conditions and scales, same microphysics?