Geomagnetic storms

Abstract

The present solar-terrestrial research has its roots in studies of geomagnetic storms, well before the discovery of the solar wind and various plasma regions in the Earth's magnetosphere. Geomagnetic storms are phenomena which originate in the solar corona and occur in the entire Sun-Earth system, including the Earth's upper atmosphere. A geomagnetic storm is defined by the existence of the main phase during which the horizontal component of the Earth's magnetic field decreases drastically. This depression, strongest at low latitudes, is thought to be caused by the enhanced ring current in the inner magnetosphere. Its growth and decay is typically monitored by the Dst index. During a geomagnetic storm, intense substorms take place successively at high latitudes. Beginning with a brief description of observational characteristics of geomagnetic storms, this chapter discusses how solar wind energy is deposited into and is dissipated in the constituent elements that are critical to magnetospheric and ionospheric processes during geomagnetic storms. Although most of the Dst variance during magnetic storms can be reproduced by changes in the electric field in the solar wind and the residuals are uncorrelated with substorms, recent satellite observations of the ring current constituents show the importance of ionospheric ions. This implies that ionospheric ions, which are associated with intense substorms, are accelerated upward and contribute to the energy density of the storm-time ring current. It is thus important to identify the role of substorm occurrence in the enhancement of magnetospheric convection driven by solar wind electric field. In evaluating the contribution of various current systems in the Dst decreases during geomagnetic storms, we contend that not only the ring current but also the tail current is important. © 2007 Springer-Verlag.