The Significance of Dipole Tilt for Substorm Onsets

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The Significance of Dipole Tilt
for Substorm Onsets

• James Wanliss

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Introduction

• What is a magnetic dipole?
• What is the magnetosphere?
• What is a magnetospheric substorm?

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Magnetic dipole
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Space Physics Substorms
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Polar substorms
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Scientific questions

• Where does substorm onset occur?
• One idea is that onset of reconnection initiates
  auroral substorms (20-30 Re)
• Another idea is that substorms are ignited by an
  instability between 6-10 Re
• In-situ magnetotail observations not definitive
• What role do parameters such as dipole tilt play?

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Data

• Results for 30 substorms via CANOPUS
  magnetometers, photometers
• Considered only substorms 90 minutes around
  local midnight
• Selection criteria
• Brightening of most equatorward auroral arc,
  immediately followed by rapid poleward motion
• Magnetic bay, and near-simultaneous Pi2 magnetic
  pulsations

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Procedure

• Select appropriate subset from original dataset
  of over 200 substorms Wanliss et al., 2001,
  2002
• Locate ionospheric footprint of ignition
• Use magnetic field models to map footprint from
  ionosphere to magnetic field lines furthest
  radial distance this determines a source
  location

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Model Issues

• Statistical models of Tsyganenko and
  collaborators (T87, T89, T96, T01) suited to this
  kind of problem Tsyganenko, 1987, 1989, 1996,
  2002ab
• T87, T89 depend only on tilt and Kp
• T96, T01 dependent on tilt, Dst, and solar wind
  Pdyn, B, V
• T01 is only model explicitly designed for inner
  tail

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Substorm example

• November 15, 1992
• Onset at 0533 UT
• Onset arc at 67.2o
• Model
• Ignition
• location

T87 T89 T96 T01
X -6.20 -7.60 -9.90 -9.79
Y 1.78 2.26 3.51 3.09
Z -2.16 -3.15 -3.82 -3.68
R 6.80 8.53 11.18 10.91
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1992/11/15
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1992/11/15
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1992/11/15
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1992/11/15
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486.1 nm
1992/11/15
Latitude
557.7 nm
onset
630.0 nm
Time (UT)
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1992/11/15 Model Fits
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Statistical onset results

• 77 of sample onsets on duskside (Figure S1)
• As Kp/Pdyn/Bz increases onsets occur closer to
  Earth (Figures S3, S4, S5)
• No dependence of onset distance on MLT (Figure
  S6)
• Strong dependence of onset Y-location on MLT
  substorms prefer duskside (Figure S7)
• Onsets occur closer for large ve dipole tilts
  (Figure S8)

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Figure S1
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Figure S2
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Figure S3
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Figure S4
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Figure S5
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Figure S6
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Figure S7
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Figure S8
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Summary

• T87/T89 have all substorms in the inner tail
• T96 has a wide range of ignition sites (negative
  Bz causes onset further downtail)
• No dependence on dipole tilt for T87/T89
• Strong dependence for T96/T01 linear inside 15 RE

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Table 1. Statistical means
Mean all substorms X Y Z R Min R Max R
T87 -6.66 1.43 -2.24 7.31 5.30 16.04
T89 -8.09 1.97 -3.02 9.06 6.04 18.41
T96 -18.59 3.42 -3.03 19.85 5.87 65.78
T01 -13.02 2.76 -3.16 14.10 5.48 29.26
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Conclusions

• Clear onset dependencies on tilt/Pdyn/Bz/Kp for
  T96, T01. Less clear for T89 not clear for T87
• For T96/T01 there is a strong linear dependence
  between tilt and downtail distance as tilt
  becomes less negative onsets occur further
  downtail
• Ignition occurs preferentially before midnight
• Best estimate (T01) for onset location is on
  duskside at a downtail distance of R14.10 RE

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References

• Tsyganenko, N. A., Global Quantitative Models of
  the Geomagnetic Field in a Cislunar Magnetosphere
  for Different Disturbance Levels, Planet. Space.
  Sci., 35, 1347-1358, 1987.
• Tsyganenko, N. A., A magnetospheric magnetic
  field model with a warped tail current sheet,
  Planet. Space. Sci., 37, 5-20, 1989.
• Tsyganenko, N. A., Effects of the solar wind
  conditions on the global magnetospheric
  configuration as deduced from data-based models,
  in Proceedings Third International Conference on
  Substorms (ICS-3), ed. E. J. Rolfe and B.
  Kaldeich, European Space Agency Spec. Publ., ESA
  SP-399, 181-185, 1996.
• Tsyganenko, N.A., A model of the near
  magnetosphere with a dawn-dusk asymmetry 1.
  Mathematical structure, J. Geophys. Res., 107,
  10.1029/2001JA000219, 2002a.
• Tsyganenko, N.A., A model of the near
  magnetosphere with a dawn-dusk asymmetry 2.
  Parameterization and fitting to observations, J.
  Geophys. Res., 107,10.1029/2001JA000220 , 2002b.

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