The Significance of Dipole Tilt for Substorm Onsets

James Wanliss

Introduction

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

Magnetic dipole

Space Physics: Substorms

Polar substorms

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?

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

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

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

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

1992/11/15

1992/11/15

1992/11/15

1992/11/15

Lat

itud

e

Time (UT)

630.

0 nm

557.

7 nm

486.

1 nm

onset

1992/11/15

1992/11/15 Model Fits

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)

Figure S1

Figure S2

Figure S3

Figure S4

Figure S5

Figure S6

Figure S7

Figure S8

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

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

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

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.