The Bimodal Solar Wind-Magnetosphere-Ionosphere System George Siscoe Center for Space Physics Boston University

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The Bimodal Solar Wind-Magnetosphere-Ionosphere
SystemGeorge SiscoeCenter for Space
PhysicsBoston University

• Vasyliunas Dichotomization
• Momentum transfer via dipole interaction
• Momentum transfer via atmospheric drag
• Dipole Interaction Regime
• No effect on neutral atmosphere
• Transpolar potential proportional to IEF
• Dayside compression
• Atmospheric Drag Regime
• Cause of neutral flywheel
• Transpolar potential saturation
• Dayside rarefaction
• Magnetopause erosion
• Summary
• Dichotomization, transpolar potential
  saturation, dayside compression versus
  rarefaction, magnetopause erosion, and neutral
  flywheel all part of one story

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Vasyliunas Dichotomization
Vasyliunas (2004) divided magnetospheres into
solar wind dominated and ionosphere dominated
depending on whether the magnetic pressure
generated by the reconnection-driven ionospheric
current is, respectively, less than or greater
than the solar wind ram pressure. The
operative criterion is

• ?o?PVAe 1
• P ionospheric Pedersen conductance
• VA Alfvén speed in the solar wind
• e magnetic reconnection efficiency

Key Point
By this criterion, the standard magnetosphere is
solar wind dominated the storm-time
magnetosphere, ionosphere dominated.
Lindsay et al., 1995
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Alternative Nomenclature
Based on current systems, Vasyliunas two cases
correspond to Chapman-Ferraro domination and
region 1 domination.
Based on the method of momentum transfer between
the solar wind and the terrestrial system, they
correspond to dipole interaction dominated and
atmospheric drag dominated
To emphasize their dynamical difference, we
choose dipole interaction and atmospheric
drag to distinguish them.
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Pertinent Properties of Dipole Interaction
Chapman-Ferraro Current System
ICF BSS Zn.p./?o ? 3.5 MA
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Ram Pressure Contribution to Dst
A dipole interaction property
Psw compresses the magnetosphere and Increases
the magnetic field on the dayside.
Chapman-Ferraro Compression
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Interplanetary Electric Field Determines Transpola
r Potential
A magnetopause reconnection property

• Magnetopause reconnection
• Equals transpolar potential
• Transpolar potential varies linarly with Ey
  (Boyle et al., 1997)
• Magnetosphere a voltage source as seen by
  ionosphere

IMF (0, 0, -5) nT
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Dipole Interaction Dominated Magnetosphere
Summary

• Psw compresses the magnetospheric field and
  increases Dst.
• Ey increases the transpolar potential linearly.
• Magnetosphere a voltage source

Key Point
Field compression and linearity of response to Ey
hold foronly one of the two modes of
magnetospheric responsesto solar wind
driversthe usual one.
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Then Came Field-Aligned Currents
Question How do you self-consistently accommodate
the extra 2 MA?
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Answer You Dont. You replace the
Chapman-Ferraro current with it.
IMF (0, 0, -5) nT
This is the usual case
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Pure Region 1 Current System
IMF (0, 0, -20) nT
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Region 1 Current System Fills Magnetopause
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Net Force on Terrestrial System
Integrate x-component of momentum stress tensor
over a surface containing the terrestrial system
S ?VV p I B2/2µo I - BB/µo
Net Force 1.2x108 N
Net Force 2.4x107 N
IMF (0, 0, -20) nT
IMF (0, 0, 0) nT
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Drag Amplification
Back of the envelope estimate
i.e., roughly an order of magnitude amplification
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Region 1 Force on the Atmosphere
IMF (0, 0, -20) nT
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Atmospheric Reaction

• Region 1 current gives the J in the JxB force
  that stands off the solar wind
• And communicates the force to the ionosphere
• Which communicates it (amplified) to the neutral
  atmosphere as the flywheel effect
• Sometimes more than 200 m/s in the E region

Richmond et al., 2003
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Elementary Dynamics

• The force on the neutral atmosphere is total
  region 1 current times polar magnetic field
  strength times length across polar cap or
  (qualitatively) I1xBPxl
• The mass of the atmosphere in and above the E
  region over the polar cap 1010 kg.
• This gives an acceleration of 7 m/s/hr/MA
• For example, 5 MA region 1 current applied for 10
  hours gives a speed of 350 m/s in the E region
  for the flywheel

Key Point
In establishing the neutral flywheel, duration of
current might count for more than strength of ram
pressure.
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Other Properties of Pure Atmospheric Drag Coupling

• Most region 1 current closes on bow shock (Alfvén
  wings)
• Reason small field strength difference between
  tail and magnetosheath
• Low-latitude cusp and equatorial dimple

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Dayside Magnetic Decompression
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Transpolar Potential Saturation
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Transpolar Potential Saturation
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Evidence of Two Coupling Modes

• Transpolar potential saturation
• Instead of this
• You have this
• Reduced dayside compression seen at synchronous
  orbit
• Instead of this
• You have this

?B erosion contribution to Btot
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The Bimodal SWMIA System
Summary

• Dipole Interaction Dominant
• Dominant current system Chapman-Ferraro
• Magnetopause current closes on magnetopause
• Magnetopause a bullet-shaped quasi-tangential
  discontinuity
• Force transfer by dipole Interaction
• Transpolar potential proportional to IEF
• Solar wind a voltage source for ionosphere
• Compression strengthensdayside magnetic field
• Minor magnetosphere erosion

• Atmospheric Drag Dominant
• Dominant current system Region 1
• Magnetopause current closesthrough ionosphere
  and bow shock
• Magnetopause a system of MHDwaves with a dimple
• Force transfer by atmospheric dragDrag
  amplification and neutral flywheel
• Transpolar potential saturates
• Solar wind a current source for ionosphere
• Stretching weakens daysidemagnetic field
• Major magnetosphere erosion

Dichotomization, transpolar potential saturation,
weak Dst response to ram pressure, magnetopause
erosion, neutral flywheel effect all part of one
story.
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THE END Thank You