The Significance of Dipole Tilt for Substorm Onsets James Wanliss.
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Transcript of The Significance of Dipole Tilt for Substorm Onsets James Wanliss.
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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 ReIn-situ magnetotail observations not definitiveWhat role do parameters such as dipole tilt play?
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Data
Results for 30 substorms via CANOPUS magnetometers, photometersConsidered only substorms ~90 minutes around local midnightSelection criteria:
1. Brightening of most equatorward auroral arc, immediately followed by rapid poleward motion
2. 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 ignitionUse 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 tilttilt and KpKpT96, T01 dependent on tilttilt, DstDst, and solar wind PPdyndyn, B, VT01 is only model explicitly designed for inner tail
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Substorm example
November 15, 1992Onset at 05:33 UTOnset arc at ~67.2o
Model
Ignition
location
T87 T89 T96 T01X -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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Lat
itud
e
Time (UT)
630.
0 nm
557.
7 nm
486.
1 nm
onset
1992/11/15
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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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SummaryT87/T89 have all substorms in the inner tailT96 has a wide range of ignition sites (negative Bz causes onset further downtail)No dependence on dipole tiltdipole tilt for T87/T89Strong dependence for T96/T01; linear inside 15 RE
87 0.2406 30.73; r 0.10T GSMX
89 0.4484 32.49; r -0.26T GSMX
96 0.6290 36.08; r -0.83T GSMX
01 0.6133 35.66; r -0.90T GSMX
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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 T87For T96/T01 there is a strong linear dependence between tilt and downtail distance; as tilt becomes less negative onsets occur further downtailIgnition occurs preferentially before midnightBest estimate (T01) for onset location is on duskside at a downtail distance of R=14.10 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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