During a magnetospheric substorm the cross-tail current is disrupted (e.g., Fairfield and Ness, 1970). A current system called the substorm current wedge (SCW; Atkinson, 1967; McPherron et al., 1973) is formed as this current is diverted through a circuit consisting of earthward (downward) field-aligned currents (FACs) on the eastern side of the wedge, a westward auroral electrojet in the ionosphere, and tailward (upward) FACs on the western side of the wedge. The current disruption leading to SCW formation is acting on current sheet that has been enhanced and/or propagated closer to Earth during the preceeding substorm growth phase (Kaufmann, 1987). The mechanism creating the disruption is still unknown (see, e.g., Lui, 1991). However, some recent models explain it via braking of the Earthward high speed plasma flows observed during the substorm expansion phase. SCW leads to Pi 2 pulsations at high-latitudes.
At the onset, the region of disrupted current in the magnetosphere is limited, but it expands with time both longitudinally and radially, away from Earth (Lopez and Lui, 1990; recently Ohtani (1998) reported also Earthward expansion). Because the current wedge signatures are both in the downward and upward FAC region, they are both rather localized, although the downward part is usually assumed to be more wide-spread. Observations show that the initially narrow SCW closes in the ionosphere via the westward travelling surge, WTS, a westward termination of the auroral bulge (Lühr and Buchert, 1988; Lopez et al., 1990). A couple of minutes after the onset, the ionospheric FAC regions are separated by 500-1000 km. During the expansion phase, the center of the downward FAC propagates eastward with the auroral bulge. The current in the upward FAC region is carried by precipitating keV electrons. Because the ions that will be carrying the downward current are slow, the current is initially carried by cold (few eV) ionospheric electrons. These electrons can be observed indirectly be the small amount of red 630.0 nm emission they produce on their way up (Opgenoorth et al., 1980; Pellinen et al., 1995; see the discussion on red aurora in connection with low-latitude aurorae).
Using ground based magnetometer data, the evolution of the SCW can be monitored. Historically a term DP 1 (disturbance polar of the first type) has been used for the expansion phase current system due to the westward auroral (or substorm) electrojet (see, e.g., Clauer and Kamide, 1985). Algorithms to calculate some important SCW characteristics have been developed both for midlatitude data (Vagina et al., 1996; Sergeev et al., 1996) and auroral latitude data (Cramoysan et al., 1995).
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