Magnetic reconnection at the Earths magnetopause: CLUSTER spacecraft observations at different scale

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Magnetic reconnection at the Earths
magnetopauseCLUSTER spacecraft observations at
different scales

• Licentiate thesis
• by
• Alessandro Retinò
• Department of Astronomy and Space Physics
• Uppsala University

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Motivation

• Magnetic reconnection - an universal mechanism to
  transfer mass, momentum and energy across
  boundary layers in plasmas
• Earths 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

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Outline

• The solar wind - Earth interaction
• Magnetic reconnection basics
• Reconnection at the Earths magnetopause
• The Cluster mission
• My contribute
• Paper I - large scales
• Paper II - small scales

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The Earths magnetosphere
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Magnetic reconnection in the magnetosphere
DR
DR
Day (2001)
IMF BZlt0
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Magnetic reconnectionLaboratory, Sun,
Astrophysics
Solar flares
Hall effect during magnetic reconnection in a
laboratory plasma Ren (2005)
Accretion disks astrophysical jets
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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

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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

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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

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Magnetic reconnectionTheoretical models
inflow
outflow
Parker (1957), Sweet (1958)
Petscheck (1964)

• Reconnection rate u0/uA0 Bn/B0
• Alfvenic outflow ueuA0
• Energy conversion WB ½ WK ½ WT
• Reconnection rate too small!

• Faster reconnection
• Smaller diffusion region
• Particle accelerate at shocks (separatrices)

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Evidence at large scales Fluid

• Walén test
• (tangential balance stress)
• Direct method
• ?ut ?Bt /(µ0?)1/2
• deHoffmann-Teller (HT) frame
• u-uHT uA

Treumann Baumjohann (1996)
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Evidence at large scales Kinetic

• Magnetosheath boundary
• layer (MSBL)
• Transmitted magnetospheric
• Incident magnetosheath
• Reflected magnetosheath

• Magnetospheric boundary
• layer (BL)
• Transmitted magnetosheath with uigtuHT
• Incident magnetospheric
• Reflected magnetospheric

Cowley (1995)
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Evidence at small scales

• Ion diffusion region
• Electron diffusion region
• Magnetic separatrices

Mozer (2002)
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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)

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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
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Paper IOverview

• duskside MP
• southern emisphere
• tailward of the cusp
• northward IMF

IMF

• orbit SC configuration ideal
• Large MP crossings over 4h
• SC/3 local monitor in MSH

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Paper IReconnection continuous for northward IMF

• in-situ at MP
• 4 hours continuous
• northward IMF, By variable

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Paper IComponent reconnection

• measure magnetic
• shear at X-line
• jet reversals ? close to X-line
• magnetic shear at X-line
• 180º antiparallel
• lt180º component

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Summary Paper I Large scales

• Magnetic reconnection continuous for hours under
  northward IMF
• In-situ evidence of component reconnection

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Paper IIMotivation

• Microphysics of reconnection
• Few detailed observations
• Separatrices close to the X-line

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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 lt 60?sh,i 3000 km (from
  comparison with simulations)

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Paper IIThe separatrix region (SR)

• magnetic separatrix identified as boundary in
  waves
• SR 5 ?sh,i wide
• inside SR few subregions ?sh,i wide
• ESW at the boundary with jet but not inside the
  separatrix region
• µ-FTE

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2
1
bulge
ESW
jet
magnetic separatrix
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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
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Paper IIA sketch of the separatrix region

• SR and its subregions spatial structures
• bulge (µ-FTE) temporal structure

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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
  one ion 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.

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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?