Interpretation of complicated discrete arc structure and behavior in terms of multiple X lines

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Abstract
In a neutral sheet with a positive Bz, X lines and O lines occur in pairs. If it is assumed that discrete auroral arcs are the mapping along field lines of X line‐O line pairs, then much of the complicated structure and behavior of discrete arcs can be readily explained. We apply this model to representative events in nightside data from a meridian‐scanning photometer chain showing the growth, expansive, and recovery phases of two substorms, all‐sky imager data, and dayside data including Viking UV images and also to a Defense Meteorological Satellite Program photograph of auroral activity. Features explained include the appearance, brightening, fading, disappearance, dividing of one arc into two, the uniting of two arcs into one, branching of arcs, two classes of short‐lived rapidly propagating (one poleward, the other equatorward) arcs, a newly identified basic substorm intensification structure, and the reaction of two arcs to a westward traveling surge. All these features can be explained in terms of forward or reverse merging at one or more X lines. Forward merging is defined as the usual flow direction for nightside merging, and reverse merging is with the flow through the X line reversed. Which is occurring at a particular time at a given X line depends on energy considerations (that is, boundary conditions). The motion of an arc is the resultant of a poleward velocity due to flux transfer through the X line and arc and the large‐scale equatorward convection (E × B/B² drift). Magnetic islands which become entrapped in dipolar fields dissipate by fast merging with rapid motion of the island and associated arc. These explain the short‐lived rapidly propagating arcs. On the nightside these islands are a likely explanation of substorm injection events.