Geomagnetic activity can be divided into two main categories, substorms and storms. Auroral substorm is a spectacle people living at auroral latitudes can enjoy during winter months (e.g., Akasofu, 1964; Elphinstone et al., 1996). At the onset of a substorm a stable, east-west oriented, southward moving auroral arc "explodes" and forms the so-called auroral bulge. In addition to these optical signatures, auroral substorms can be seen in the ground based magnetometer recondings due to created ionospheric currents (see AE index). However, the substorm process has its origin in the Earth's magnetosphere, where major phenomana occur during this time (see substorm phases below). We talk thus about magnetospheric substorms. There are several substorm models, and many substorm related questions are still under (sometimes) heated debate.
Note that some substorms never "breakup" into full strength: these events are called pseudobreakups. On the other hand, the strongest substorms tend to occur during longer periods of enhanced activity, i.e., magnetic storms.
According to the many phenomenological features, substorms have been divided into three phases (Rostoker et al., 1980):
Note that it is often difficult to define these phases rigorously, especially during strong and/or complex activity.
The energy for the substorms originate from the solar wind (see, e.g., Nishida, 1983). The physical processes involved in dissipating this energy have been divided into directly driven and loading-unloading processes (Rostoker et al., 1987; Baker et al., 1997b). The driven process dissipates energy globally and continuously. Some studies have indicated that even this global system may react to the substorm onset (Opgenoorth and Pellinen, ICS-4 meeting, 1998). The loading-unloading processes deal directly with the substorms, dividing them into three phases mentioned above. During the growth phase, energy is stored within the magnetosphere as excess magnetic flux in the tail lobes (see reconnection). This energy is dissipated explosively during the substorm expansion phase into magnetosphere and ionosphere (e.g., Baker et al., 1997a). The energy put into the plasmoids returns eventually to the solar wind.
- Akasofu, S.-I., The development of the auroral substorm, Planet. Space Sci., 12, 273-282, 1964.
- Baker, D. N., T. I. Pulkkinen, M. Hesse, and R. L. McPherron, A quantitative assessment of energy storage and release in the Earth's magnetotail, J. Geophys. Res., 102, 7159-7168, 1997.
- Baker, D. N., A. J. Klimas, D. Vassiliadis, T. I. Pulkkinen, and R. L. McPherron, Re-examination of driven and unloading aspects of magnetospheric substorms, J. Geophys. Res., 102, 7169-7177, 1997.
- Elphinstone, R. D., J. S. Murphree, and L. L. Cogger, What is a global auroral substorm?, Rev. of Geophys., 34, 169-232, 1996.
- Nishida, A., IMF control of the Earth's magnetosphere, Space Sci. Rev., 34, 185, 1983.
- Rostoker, G., S.-I. Akasofu, J. Foster, R. A. Greenwald, Y. Kamide, K. Kawasaki, A. T. Y. Lui, R. L. McPherron, and C. T. Russell, Magnetospheric substorms - Definition and signatures, J. Geophys. Res., 85, 1663-1668, 1980.
- Rostoker, G., S.-I. Akasofu, W. Baumjohann, Y. Kamide, and R. L. McPherron, The roles of direct input of energy from the solar wind and unloading of stored magnetotail energy in driven magnetospheric substorms, Space Sci. Rev., 46, 93-111, 1987.