The Utilization of Ground Magnetometer Data in Magnetospheric Physics

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The Utilization of Ground Magnetometer Data in
Magnetospheric Physics

• Professor Robert L. McPherron
• Institute of Geophysics and Planetary Physics
• University of California Los Angeles
• Invited Lecture Presented at
• XIIIth IAGA Workshop on Observatory Instruments,
  Data Acquisition, and Processing
• June 16, 2008 - Golden, Co

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Outline

• The ionospheric quiet day current
• The magnetopause current
• The ring current
• Tangential drag and the magnetotail
• Magnetospheric convection
• Region 1 region-2 currents
• The DP-2 current
• The DP-1 current
• The substorm current wedge
• The partial ring current
• The tightly coupled magnetospheric system

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The Ionospheric Dynamo - Sq

• Solar illumination creates a hot spot in
  ionosphere near local noon
• Ionized atmosphere flows away from region of high
  pressure across the magnetic field
• Moving a conductor in a magnetic field produces a
  current (dynamo)
• The current produces the quiet day magnetic
  variation Sq
• The pattern of this current is shown in the
  diagram

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Example Quiet Day Variation
Noon
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The Magnetopause Current

• Electrons and protons in the solar wind turn in
  the dipole field producing a sheet of current in
  space called the magnetopause
• The current opposes the Earths field outside the
  current sheet and increases it inside completely
  enclosing the Earths field
• The current circulates CCW in northern hemisphere
  and has effects similar to Sq

K
Image from Siscoe
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The Ring Current - Dst

• The ring current is produced by ions drifting
  westward and electrons drifting eastward in
  dipole field about 4 Re from Earth
• Their effect on ground is a southward magnetic
  field that decreases the intensity of main field
• The magnitude of the disturbance is proportional
  to total energy of drifting particles

west
Positive ions drift westward and electrons drift
eastward creating a westward current
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Example of Ring Current Effects

• Coronal mass ejection produce intervals of strong
  southward Bz at the earth
• Magnetic reconnection drives magnetospheric
  convection
• Convection drives currents along field lines and
  through ionosphere
• Ground magnetometers record effects of
  ionospheric and magnetospheric currents in H and
  other components
• H traces are used to construct the AE and Dst
  index

B
Bz
AE
Dst
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The Tail Current

• The tail current is produced by two solenoids
  downstream of Earth with current flowing in
  opposite sense in each solenoid
• The effect is a fringing field in the vicinity of
  the Earth that reduces the horizontal component
• The effect is stronger on night and evening side
  creating an asymmetry in the surface field

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An Empirical Fit to Magnetic Field Data

• Use the entire history of observations of the
  magnetospheric magnetic field
• Create a spherical harmonic fit to the
  observations
• Include dipole tilt as function of season
• External field is reconnecting with dipole field

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Two Forms of Tangential Drag on Magnetosphere

• Viscous interaction moves closed field lines
  antisunward in a boundary layer
• Magnetic reconnection connects dipole field lines
  to the IMF and moves open field lines across the
  poles
• Both open and closed field lines must return to
  the dayside else flux imbalance will occur

Cowley, S.W.H., The causes of convection in the
earth's magnetosphere A review of developments
during the IMS, Reviews of Geophysics and Space
Physics, 20(3), 531-565, 1982.
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Generation of Region-1 Field Aligned Currents
The motion of plasma in the low latitude boundary
layer and its return to dayside produces an
electric field Charges accumulate on the
boundaries of the region, positive at the inner
edge at dawn and negative at inner edge at
dusk This produces a magnetospheric electric
field from dawn to dusk Field lines are nearly
perfect conductors and transmit the field to the
ionosphere driving current
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The Hall Current (DP-2)

• In the presence of orthogonal E and B fields both
  electrons and ions drift according to
• The E x B drift is from noon towards midnight and
  corresponds to the motion of the feet of flux
  tubes convecting in the magnetosphere
• Because ions are more likely to collide with
  neutral atoms they drift slower than the
  electrons
• Consequently there is a net current opposite to
  the direction of drift. This is the Hall current.

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The DP-2 Equivalent Current System

• Magnetospheric convection in a 2-cell pattern
  produces a closed circuit of sheet current in the
  ionosphere called DP-2
• The current flows from midnight (bottom) towards
  noon (yellow) and returns along the auroral oval
• High conductivity in the auroral oval
  concentrates the current into the eastward (left)
  and westward (right) electrojets

Clauer, C. R., and Y. Kamide (1985), DP 1 and DP
2 current systems for the March 22, 1979
substorms, J. Geophys. Res., 90(A2), 1343-1354.
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Effects of the Auroral Electrojets

• Magnetometers below the eastward electrojet
  measure positive (northward) perturbations
• Below the eastward electrojet they measure
  negative (southward) perturbations
• The time scale of the electrojet disturbances is
  about 3 hours and repeats several times per day

Kamide, Y., ET AL., Ground-Based Studies of
Ionospheric Convection Associate with Substorm
Expansion, J. Geophys. Res., 99(A10),
19451-19466.
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Generation of Region-2 Field Aligned Currents

• The low latitude boundary layer moves tailward
  just inside the magnetopause
• The electric field within the dawn side layer
  points outward
• Just inside the boundary layer the plasma flows
  Sunward so the electric field is inward
• Magnetic field lines map the magnetospheric
  electric fields to the ionosphere
• On the dawn side the E field point poleward
  across the polar cap and equatorward
• The electric fields drive currents in the
  ionosphere that diverge at the point where the
  inner edge of the boundary layer maps
• The current divergence is fed by a field-aligned
  current called the Region 1 current

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The DP-1 Equivalent Current System

• The DP-1 current system flows westward from dawn
  to midnight (red) and is strongest premidnight
• This system is added to the DP-2 system which
  starts about an hour earlier

Clauer, C. R., and Y. Kamide (1985), DP 1 and DP
2 current systems for the March 22, 1979
substorms, J. Geophys. Res., 90(A2), 1343-1354.
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The Substorm Current Wedge

• The DP-1 current system is only an ionospheric
  equivalent. In reality it is a three dimensional
  system with tail current flowing down field lines
  through the ionosphere then out to the tail
• This current forms after the onset of a substorm
  expansion phase and persists for about 30 minutes
• This current causes the sudden onset of negative
  pertubations near midnight at high latitudes

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Midlatitude Effects of theSubstorm Current Wedge

• Midlatitude stations are more strongly affected
  by the field-aligned portion of the current wedge
  than by the ionospheric segment
• The H component perturbation is positive inside
  the wedge and weakly negative outside
• The D component perturbation is antisymmetric
  about the center positive premidnight and
  negative post midnight
• The event at Tucson in red is seen east of
  central meridian of the wedge

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The Partial Ring Current

• The inner edge of the tail current and the ring
  current are stronger near dusk
• The effects of this asymmetry is represented by a
  partial ring current near dusk
• Adding this to a symmetric ring accounts for the
  more negative perturbations near dusk

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Conclusions

• Electrical currents are everywhere in the Earths
  magnetosphere and ionosphere
• These currents produce magnetic effects
  observable in ground magnetograms
• Continuous records from many stations are needed
  to study these currents and to determine how they
  are affected by the Sun and solar wind
• The World-wide collection of magnetic
  observatories is essential to understanding