The Significance of Dipole Tilt for Substorm Onsets

1. The Significance of Dipole Tilt for Substorm Onsets • James Wanliss

2. Introduction • What is a magnetic dipole? • What is the magnetosphere? • What is a magnetospheric substorm?

3. Magnetic dipole

5. Space Physics: Substorms

6. Polar substorms

7. 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?

8. 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

10. 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

11. 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

12. Substorm example • November 15, 1992 • Onset at 05:33 UT • Onset arc at ~67.2o Model Ignition location

13. 1992/11/15

14. 1992/11/15

15. 1992/11/15

16. 1992/11/15

17. 486.1 nm 1992/11/15 Latitude 557.7 nm onset 630.0 nm Time (UT)

18. 1992/11/15 Model Fits

20. 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)

21. Figure S1

22. Figure S2

23. Figure S3

24. Figure S4

25. Figure S5

26. Figure S6

27. Figure S7

28. Figure S8

29. 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

30. Table 1. Statistical means

31. 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 R=14.10 RE

32. 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.