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

This licentiate thesis by Alessandro Retinò explores the phenomenon of magnetic reconnection in Earth's magnetosphere, analyzing data from the CLUSTER spacecraft mission. The study delves into the impact of reconnection on various scales, from small diffusion regions to large volumes in space. Through detailed observations and theoretical models, the thesis sheds light on the fundamental processes involved in magnetic reconnection, including plasma dynamics and energy conversion mechanisms. By investigating both large and small scales, the research contributes to a deeper understanding of this universal mechanism in space physics.

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

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  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?

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