Variability in the north-south orientation of the interplanetary magnetic field causes episodic energy loading-dissipation cycles termed magnetospheric substorms (Baker et al., 1996; Sergeev et al., 1996b). Substorms typically occur at a rate of four to five per day, each lasting typically two to three hours. They are initiated by enhanced dayside reconnection and hence increased energy input to the magnetosphere. This causes a configuration change in the magnetosphere including enhancement of the magnetospheric current systems and formation of a thin and intense current sheet from near-geostationary distance outward (McPherron, 1970; Pulkkinen et al., 1992). The current sheet at the tail center has a thickness of only a few hundred kilometers (comparable to the thermal ion gyroradius), and the magnetic field component normal to the current sheet becomes very small, only a few nanotesla (Sergeev et al., 1993). As this thin current sheet grows unstable, a rapid dissipation process quickly expands to a large-scale reconnection event whose effects are observable over a large portion of the coupled magnetosphere-ionosphere system (Hones Jr, 1979). Because the reconnection region is limited in the cross-tail direction, the flow shear between the fast outflow from the reconnection region and the ambient Earthward plasma flow within the plasma sheet creates a pair of field-aligned current sheets, where the current flows to the ionosphere at the eastern edge of the reconnection region and out from the ionosphere at the western edge (McPherron et al., 1973). On ground, magnetospheric substorms are seen as disturbances in the geomagnetic field caused by the field-aligned current systems and auroral electrojet currents in the ionosphere (Baumjohann et al., 1981). Energetic electrons precipitating into the ionosphere and colliding with the atmospheric constituents create bright auroral displays in the night sector auroral region (Elphinstone et al., 1993). Figure 5 shows a schematic of the magnetotail reconfiguration process during a substorm.
Magnetic reconnection in the tail leads to formation of a plasma structure where the field lines are no longer connected to the Earth. This plasmoid is accelerated tailward and ejects a large portion of the plasma sheet plasma, magnetic flux, and energy back to the solar wind (Hones Jr, 1979). It is estimated that about half of the energy that enters via the dayside reconnection process is processed in the inner magnetosphere and ionosphere, while the other half is carried by the plasmoid(s) back to the solar wind (Ieda et al., 1998).
Magnetospheric substorms require a period of enhanced energy input (southward interplanetary field) from 30 minutes to 1 hour. If the energy input continues significantly longer (≥3 h), a magnetic storm develops (Gonzalez et al., 1994). As it takes several hours of southward interplanetary field and/or high solar wind speed to create a magnetic storm, they often follow from the interaction of a fast solar wind stream, an interplanetary magnetic cloud, an ICME, or other coherent solar wind structure. Magnetic storms typically last from about 12 hours to a few days. Storms are characterized by the formation of an intense ring current encircling the Earth with current peak at about 4 RE, i.e., well inside the geostationary orbit. The ring current is populated both by efficient convection and injection of plasma sheet particles into the inner region and by strongly enhanced ion outflow from the ionosphere. At times, the ionospheric outflow can be strong enough for the ionospheric oxygen to dominate the energy density in the ring current (Daglis, 1997). Storms as largest magnetospheric disturbances are associated with many of the space weather phenomena.
While substorms can occur without magnetic storms, almost all storms include also substorm activity. The interaction of the inner magnetosphere storm-associated processes with the magnetotail substorm-associated processes is highly complex, and presently under active study. Especially, it is debated whether the substorm-associated magnetic field variations and the associated (inductive) electric fields are necessary for the buildup of the ring current energy density (McPherron, 1997; Daglis and Kamide, 2003). While some researchers argue that convection alone can account for the ring current increase (Ebihara and Ejiri, 2003), others have concluded that especially the energization process to energies exceeding 100 keV needs smaller-scale, time-varying electric fields (Ganushkina et al., 2005).
The magnetospheric substorms and storms are the most commonly occurring responses to enhanced solar wind driving, but the magnetosphere can enter into a variety of other dynamic states. Steady convection intervals (SMC) are periods of steadily southward IMF and steady, relatively slow solar wind flow that drive continuous, low-level auroral activity without evident substorm expansion phase activity (Sergeev et al., 1996a). On the other hand, stronger levels of driving may lead to sawtooth oscillations, which are large-scale substorms associated with strong, longitudinally extended, quasi-simultaneous injections at geostationary orbit recurring every 2 – 3 h (Henderson et al., 2006; Pulkkinen et al., 2006). Magnetic storms include, in addition to strong substorm activity, also other kinds of strong activity, field-aligned currents, and inner magnetosphere disturbances (McPherron, 1997).
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