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 Re In-situ magnetotail observations not definitive What role do parameters such as dipole tilt play?

Data Results for 30 substorms via CANOPUS magnetometers, photometers Considered only substorms ~90 minutes around local midnight Selection 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 ignition Use 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] tiltKp T87, T89 depend only on tilt and Kp tiltDst P dyn T96, T01 dependent on tilt, Dst, and solar wind P dyn, B, V T01 is only model explicitly designed for inner tail

Substorm example November 15, 1992 Onset at 05:33 UT Onset arc at ~67.2 o Model Ignition location T87T89T96T01 X Y Z R

1992/11/15

Latitude Time (UT) nm nm nm onset 1992/11/15

1992/11/15 Model Fits

Statistical onset results 77% of sample onsets on duskside (Figure S1) As Kp/P dyn /B z 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

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

Table 1. Statistical means Mean all substorms XYZRMin RMax R T T T T

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 R E

References Tsyganenko, N. A., Global Quantitative Models of the Geomagnetic Field in a Cislunar Magnetosphere for Different Disturbance Levels, Planet. Space. Sci., 35, , Tsyganenko, N. A., A magnetospheric magnetic field model with a warped tail current sheet, Planet. Space. Sci., 37, 5-20, 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, , Tsyganenko, N.A., A model of the near magnetosphere with a dawn-dusk asymmetry 1. Mathematical structure, J. Geophys. Res., 107, /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, /2001JA000220, 2002b.