Ionospheric%20Current%20and%20Aurora

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Ionospheric Current and Aurora
CSI 662 / ASTR 769 Lect. 12
Spring 2007 April 24, 2007

• References
• Prolss Chap. 7.1-7.6, P349-379 (main)
• Tascione Chap. 8, P. 99 112 (supplement)

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Topics

• Polar Upper Atmosphere
• Ionospheric Currents
• Aurorae
• Ionosphere and magnetosphere coupling

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Ionosphere Currents
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Fast and Slow Wind
Polar Upper Atmosphere

• Polar Cap 30
• Polar oval a few degree
• Subpolar latitude

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Fast and Slow Wind
Polar Upper Atmosphere

• Magnetic field connection
• Polar Cap magnetotail lobe region, open
• Polar oval plasma sheet, open
• Subpolar latitude conjugate dipole field, closed

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Fast and Slow Wind
Convection and Electric Field

• Polar cap electric field Epc
• Dawn to dusk direction
• Epc 10 mV/m
• Polar cap potential 30 kV from 6 LT to 18 LT,
  over 3000 km

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Fast and Slow Wind
Convection and Electric Field

• Polar cap electric field originates from solar
  wind dynamo electric field
• Same direction
• Same overall electric potential drop
• Electric field is 40 times as strong as in
  solar wind

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Convection and Electric Field

• Polar cap convection
• Caused by EXB drift
• anti-sunward
• Drift time scale cross the polar cap 2 hours

Drift velocity 500 m/s, when E10 mV/m,
and B20000 nT
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Fast and Slow Wind
Convection and Electric Field

• Polar oval electric field Eo
• Dusk to dawn direction, opposite to polar cap
  field
• E0 30 mV/m
• Counter-balance the polar cap field
• Polar oval convection
• Sunward convection
• Form a close loop with the polar cap convection
• Two convection cells

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Fast and Slow Wind
Convection and Electric Field

• Polar oval electric field Eo
• Dusk to dawn direction, opposite to polar cap
  field
• E0 30 mV/m
• Counter-balance the polar cap field
• Polar oval convection
• Sunward convection
• Form a close loop with the polar cap convection
• Two convection cells

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Fast and Slow Wind
Ionosphere Current

• Pederson current perpendicular B, parallel E
  horizontal
• Hall current perpendicular B,
  perpendicular E horizontal
• Burkeland current parallel to B vertical

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Fast and Slow Wind
Ionosphere Current

• Birkeland current Field-aligned current
• Region 1 current on the poleward side of the
  polar oval
• Region 2 current on the equatorward side of the
  polar oval

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Fast and Slow Wind
Ionosphere Current

• Pederson current flows from dawn to dusk in the
  polar cap
• Pederson current flows radially in the polar
  oval, dusk to dawn
• Pederson current forms a closed loop with
  Burkeland currents in the two boundary regions
  region 1 and 2
• Hall current direction is opposite to the
  convection, because ions drift slower than the
  electrons
• Westward at the dawn sector
• Eastward at the dusk sector

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Fast and Slow Wind
Ionosphere Conductivity

• Deriving conductivity s is to find the drift
  velocity under the E in the three components
• Birkeland s parallel to B
• Pederson s parallel to E, E per B
• Hall s per E and B

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Fast and Slow Wind
Ionosphere Conductivity
Parallel conductivity
Force equilibrium Electric force frictional
force No Lorentz force
For plasmas (without neutral), Coulomb collision
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Fast and Slow Wind
Ionosphere Conductivity
Transverse conductivity
Force equilibrium Electric force magnetic
force frictional force
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Fast and Slow Wind
Ionosphere Conductivity
Transverse conductivity
Maximum conductivity Transverse conductivity,
especially Hall, confines to a rather narrow
range of height ( 125 km), the so called dynamo
layer
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Aurora
Image taken near Richmond VA, Oct 29, 2003
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(No Transcript)
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Akasofu, Secrets of the Aurora
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Patches and Bands
Akasofu, Secrets of the Aurora
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Aurora

• Form
• Discrete arcs, bands, rays, patches
• Diffuse
• Height gt 100 km
• Orientation
• Vertical along the magnetic field line
• Horizontal primarily east-west direction
• Colors and emitting elements
• O red (630.0 nm, 630.4 nm), yellow-green (557.7
  nm)
• N2 blue-violet (391.4 nm 470 nm)
• N2 dark red (650 nm 680 nm)
• Intensity up to a few 100 kR (kilo Rayleigh)

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Aurora

• Aurorae are caused by the incidence of energetic
  particles onto the upper atmosphere
• Particles move-in along the open polar magnetic
  fields
• The particles are mostly electrons in the energy
  range of 100 ev to 10 kev.
• Ions are also observed

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Aurora Processes

• Primary collision
• Scattering (elastic collision)
• Collisional ionization
• Collisional dissociation
• Collisional excitation
• Secondary process
• Secondary ionization
• Secondary dissociation
• Secondary scattering
• Charge exchange
• Dissociation exchange
• Excitation exchange
• Dissociative recombination
• Radiative recombination
• Collisional quenching

• Energy conversion
• 1 radiation
• 50 heating
• 30 chemical energy
• Other scatter back to magnetosphere

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The Rayleigh (R) A Basic Unit for measuring
Aurora-Airglow Emissions

• One R corresponds to the emission rate of 106
  photons per second radiated isotropically from an
  atmospheric column with a base area of 1 cm2
• Brightness of the Milky Way Galaxy 1 kR

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Auroral Particles

• Not solar wind particles
• Particles are from magnetotail plasma sheet, with
  which the polar oval is magnetically connected
• Diffuse aurora
• convection and subsequent pitch angle diffusion
  of plasma sheet particles
• Discrete aurora
• Produced by higher energy electrons (Ee gt 1 keV)
• Plasma sheet electron (Ee lt 1 keV)
• Additional acceleration is needed
• Acceleration along magnetic field-aligned
  electric fields
• Double layer
• Plasma instability produces localized potential
  differences

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Ionosphere-Magnetosphere Coupling

• Region 1 current
• Magnetotail current is re-directed to the
  ionosphere
• Also produce auroral oval electrojet
• Energy is from solar wind dynamo
• Energy is dissipated in the ionosphere through
  Joule heating

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Ionosphere-Magnetosphere Coupling

• Region 2 current
• Associated magnetic field lines end in the
  equatorial plane of the dawn and dusk
  magnetosphere at a geocentric distance of L
  7-10
• Driven by excess charge in the dawn and dusk
  sectors of the dipole field, caused by different
  particle paths of electrons and ions

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Ionosphere-Magnetosphere Coupling

• Drift of particles from the plasma sheet

• At small L, curvature-gradient drift dominates
• Particles can only drift to within a certain
  distance of the dipole

• Ions and electrons drifts in different direction
  along the dipole
• There is a forbidden zone for ions (electrons)
• Excess charges accumulate

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The End