Observations of auroral substorms occurring together with preexisting “quiet time” auroral patterns

Abstract

Although horse collar configurations are commonly associated with relatively quiescent magnetospheric conditions, observations made with the Viking UV imager are presented which clearly demonstrate that classical auroral substorms can coexist with such patterns. Thus the magnetospheric conditions that lead to the production of these “quiet time” auroral patterns are not necessarily stable to the growth of the instability that leads to substorm onset. As well, the horse collar patterns provide useful natural indicators of the underlying magnetospheric topology and can be used to determine (in a relative sense) the source location of the substorm activity. We conclude that the horse collar patterns have a distant tail source location and that the substorm onsets have a source location on closed field lines with much lower L values. The observations are therefore inconsistent with the onset mechanisms proposed in either the thermal catastrophe model or the boundary layer dynamics model. In addition, we have shown that substorm‐related plasma sheet dropouts are not always produced by a neutral line induced thinning of the plasma sheet but could also be caused by the large‐scale motion of the open field line region. Plasma data from DMSP F6, DMSP F7, ISEE 1, and Viking are consistent with the interpretation that the regions between the horse collar arcs are threaded by open lobe field lines and that open field lines can also partially intrude into the space between the transpolar arcs and the lower latitude main oval. The significance of these results for substorm activity which occurs together with quiet time auroral patterns is discussed in terms of the flux rope plasmoid version of the near‐Earth neutral line model. Depending upon the location and extent of the open field line regions relative to the developing plasmoid, it is suggested that several possibilities exist for the subsequent evolution of the substorm activity in the tail and that these different scenarios can be related to specific types of auroral morphologies in the ionosphere.