Evaluation of the Cross‐Field Current Instability as a Substorm Onset Process With Auroral Bead Properties

Abstract

AbstractDetailed properties of the spatially periodic auroral beads (ABs) that develop immediately prior to auroral substorm expansion onset on breakup auroral arcs pose stringent observational constraints on the substorm onset process. Theoretical predictions of the cross‐field current instability (CCI) are evaluated in terms of these constraints. It is found that CCI satisfies these constraints associated with ABs, that is, their growth rates, wavelengths, and periods, by matching these characteristics well with the theoretical predictions of CCI. Several subtle trends in AB properties are revealed with CCI solutions applied to them. For a given observed wavelength in the ionosphere, waves with longer periods are generated at onset sites at longer downstream distances for both westward and eastward drifting ABs. Waves with higher growth rates are generated by higher drift speeds for a given observed period and for a given observed wavelength by onset sites at closer downstream distances. A distinct group of eastward drifting ABs having long periods and small wavelengths are related to onset sites with relatively high drift speeds and large downstream distances. These revelations indicate that the onset site can occur over a range of downstream distances. ABs possessing two different propagating directions may be accommodated. A two‐dimensional equilibrium current sheet can be constructed from the adopted parameters for the current sheet used in CCI solution. Some misunderstandings of CCI are clarified as well. All these findings provide strong evidence for CCI as a viable substorm onset process.