Abstract
The auroral substorm is an organized sequence of events seen in the aurora near midnight. It is a manifestation of the magnetospheric substorm which is a disturbance of the magnetosphere brought about by the solar wind transfer of magnetic flux from the dayside to the tail lobes and its return through the plasma sheet to the dayside. The most dramatic feature of the auroral substorm is the sudden brightening and poleward expansion of the aurora. Intimately associated with this expansion is a westward electrical current flowing across the bulge of expanding aurora. This current is fed by a downward field-aligned current (FAC) at its eastern edge and an upward current at its western edge. This current system is called the substorm current wedge (SCW). The SCW forms within a minute of auroral expansion. FAC are created by pressure gradients and field line bending from shears in plasma flow. Both of these are the result of pileup and diversion of plasma flows in the near-earth plasma sheet. The origins of these flows are reconnection sites further back in the tail. The auroral expansion can be explained by a combination of a change in field line mapping caused by the substorm current wedge and a tailward growth of the outer edge of the pileup region. We illustrate this scenario with a complex substorm and discuss some of the problems associated with this interpretation.
Highlights
The word “substorm” first appeared in a review of magnetic storms by Chapman (1962)
The justification for this was first made in conference proceedings (McPherron 1967) where we concluded “In order to generalize the concept of the auroral substorm to include the world-wide disturbance characteristics and to emphasize the importance of the magnetosphere in auroral zone observations, we suggest a new terminology: magnetospheric substorm.”
Panel (b) shows the total luminosity recorded by three all sky imagers (ASI) across Canada
Summary
The word “substorm” first appeared in a review of magnetic storms by Chapman (1962). “ his polar storms I call polar substorms, and denote them and their currents by DP.” The association of polar substorms, and the justification of the use of the term substorm, was further developed in three papers (Akasofu et al 1963; Akasofu and Chapman 1963a, b). The evolution of the substorm concept to include phenomena outside the ionosphere began with a paper by Jelly and Brice (1967) which noted that “...auroral precipitation is not a spatially isolated phenomenon, but is intimately connected with large-scale processes that occupy a substantial part of the magnetosphere.” They suggested that a new term “elementary magnetospheric substorm” be introduced to describe this behavior. We argued that the term was too complex and that a more appropriate term was “magnetospheric substorm.” The justification for this was first made in conference proceedings (McPherron 1967) where we concluded “In order to generalize the concept of the auroral substorm to include the world-wide disturbance characteristics and to emphasize the importance of the magnetosphere in auroral zone observations, we suggest a new terminology: magnetospheric substorm.”. We argued that the term was too complex and that a more appropriate term was “magnetospheric substorm.” The justification for this was first made in conference proceedings (McPherron 1967) where we concluded “In order to generalize the concept of the auroral substorm to include the world-wide disturbance characteristics and to emphasize the importance of the magnetosphere in auroral zone observations, we suggest a new terminology: magnetospheric substorm.” This idea was published later in Coroniti et al (1968) where we said. “...It is clear, that the auroral substorm is part of a worldwide disturbance, and that it is the dynamical processes occurring throughout the magnetosphere that determine the local-time characteristics of the substorm.” Observational support for this
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