The external substorm triggering is a complex subject. The possibility that the northward turning of the IMF (see solar wind) might initiatite a substorm was first discussed by Caan et al. (1977) and Rostoker (1983). In addition to Bz changes, also IMF By reduction can be a trigger (Troshichev et al., 1986). Most agree that some substorms seem to be triggered externally (McPherron et al., 1986; Lyons et al., 1997); however, there are also some substorms that seem not to be (Henderson et al., 1996). The convection reduction model for substorms is based on the fact that this kind of IMF changes reduce the convection electric field within Earth's magnetosphere.
It is also possible that changes in the solar wind could start a substorm (e.g., Ullaland et al., 1993; Elsen et al., ICS-4 meeting, 1998). This may be partly due to the fact that changes in IMF and solar wind occur often simultaneously. However, in the ICS-4 meeting two papers were presented where the effects of sudden solar wind density changes on the lobe field and the cross-field current during substorms were evaluated (Lockwood and Opgenoorth; Opgenoorth et al.). It seems that, at almost any time during the growth phase, a density increase could enhance the tail current or the lobe field above the level where a substorm expansion is usually triggered. An event was reported where a such a pressure change quenched a substorm that had already started! In the same meeting, Goodrich et al. reported how solar wind pressure pulses could directly modulate the auroral intensity.
It should be noted that one encounters many problems when studying external substorm triggering. These include uncertain timing of the observed signatures and the unknown spatial differences of IMF and solar wind. In addition, if the solar wind discontinuity is slanted, response time from the flank contact can differ considerably from that of the subsolar point. See Ridley et al. (1998) for a discussion on the propagation time of IMF from a satellite to the magnetopause.
Blagoveshchenskaya et al. and Troshichev (ICS-4 meeting, 1998) described an event where a substorm may have been triggered artificially with a ionospheric heater system.
- Caan, M. N., R. L. McPherron, and C. T. Russell, Characteristics of the association between the interplanetary magnetic field and substorms, J. Geophys. Res., 82, 4837-, 1977.
- Henderson, M. G., G. D. Reeves, R. D. Belian, and J. S. Murphy, Observations of magnetospheric substorms with no apparent solar wind/IMF trigger, J. Geophys. Res., 101, 10773-, 1996.
- Lyons, L. R., G. T. Blanchard, J. C. Samson, R. P. Lepping, T. Yamamoto, and T. Moretto, Coordinated observations demonstrating external substorm triggering, J. Geophys. Res., 102, 27039-27051, 1997.
- McPherron, R. L., T. Terasawa, and A. Nishida, Solar wind triggering of substorm onset, J. Geomagn. Geoelectr., 38, 1089, 1986.
- Ridley, A. J., G. Lu, C. R. Clauer, and V. O. Papitashvili, A statistical study of the ionospheric convection response to changing interplanetary magnetic field conditions using the assimilative mapping of ionospheric electrodynamics technique, J. Geophys. Res., 103, 4023-4039, 1998.
- Rostoker, G., Triggering of expansive phase intensifications of magnetospheric substorms by nortward turnings of the interplanetary magnetic field, J. Geophys. Res., 88, 6981-, 1983.
- Troshichev, O. A., A. L. Kotikov, B. D. Bolotinskaya, and V. G. Andrezen, Influence of the IMF azimuthal component on magnetospheric substorm dynamics, J. Geomagn. Geoelectr., 38, 1075, 1986.
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