POLAR Provides First Experimental Documentation of Magnetic Reconnection

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POLAR Provides First ExperimentalDocumentation
of Magnetic Reconnection
Reconnection, the fundamental process for
transferring/exchanging energy in the universe,
occurs near earth, at the sun, and at other star
systems Magnetic reconnection is no longer just
a theoretical construct with schematic
illustrations The Hydra, EFI and MFE instrument
teams, using POLAR particle and field data are
producing maps of the reconnection site
Discovery The first experimental documentation
that the process of collisionless magnetic
connection works as theoretically foreseen and
can be a quasi-stationary process
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POLAR Provides First ExperimentalDocumentation
of Magnetic Reconnection

• MEASUREMENTS
• 3.5 hours of full resolution magnetic field,
  electric field, ion and electron particle data
• 2 million pieces of information

Reconnection region will wash over POLAR as
compression occurs

• OBSERVATIONS
• POLAR orbit was tangential to the magnetopause
  current layer
• Although stationary within its own moving frame
  of reference, the motion of the reconnection site
  relative to the moving spacecraft meant that the
  spacecraft, over time, wandered in and about the
  reconnection region, sampling and resampling the
  layers.
• Superposed epoch processing of the 2 million
  pieces of information yielded experimental
  confirmation of 12 theoretical predictions
  regarding collisionless magnetic reconnection.

Position of POLAR (red) with respect to
reconnection region (yellow) is dynamic, both are
moving.
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Fingerprints of Collisionless ReconnectionMagnet
ic field geometry
Simulation based on theory
Theory predicts that the magnetic field strength,
shown in color, should approach zero at the
reconnection site. The POLAR analysis resolves
this expected magnetic field structure to a
resolution approaching 1 km At one point during
the interaction, the magnetic field strength is
comparable to uncertainty levels for the
magnetometer
toward magnetosphere
toward solar wind
Superposed epoch image of reconnection site
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Fingerprints of Collisionless ReconnectionThe
Electron Pressure Ridge
Simulation of electron pressure ridge
As material flows inward, particle density builds
and the particles are heated. If the
electrodynamics is to be steady, without
resistivity, an electron pressure ridge will
form at right angles to the inflowing
material. POLAR observes this high pressure of
electrons along the direction separating the
solar wind from the magnetosphere side
toward magnetosphere
toward solar wind
Superposed epoch image of electron pressure at
reconnection site
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Fingerprints of Collisionless ReconnectionUnmagn
etized Electrons

• A byproduct of magnetic reconnection is that
  particles initially located on a magnetic field
  line do not remain there once the reconnection
  has taken place.
• Near a neutral point, at the magnetic null,
  charged particles will no longer be constrained
  to gyromotion about the field.
• Low energy electrons, with their small
  gyroradius, are ideal tracers for this condition.
• Shown is the ratio of electron gyroradius to the
  scale length of B. When this number is gt1,
  individual electrons are no longer controlled by
  the magnetic field and are highly likely to move
  from one field line to another

toward magnetosphere
toward solar wind
Superposed epoch image of degree to which
electrons are controlled by the magnetic field
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POLAR Provides First ExperimentalDocumentation
of Magnetic Reconnection

• INTERPRETATION
• Short spatial scales are present within this
  event
• These short scales are compatible with the MHD
  description for thin current layers near a
  reconnection site expected when the ambipolar and
  Hall electric fields control the electrodynamics

• IMPLICATIONS
• Unlike the traditional view of anomalous
  resistivity leading to magnetic reconnection, the
  present work demonstrates that collisionless
  effects can provide the necessary departure from
  ideal MHD to permit reconnection to occur.