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A geometric wedge model of the near-earth nightside plasma sheet is used toderive a wave equation for low frequency shear flow-interchange waves whichtransmit $\vec{E} \times \vec{B}$ sheared zonal flows along magnetic flux tubestowards the ionosphere. Discrepancies with the wave equation result used inKalmoni et al. (2015) for shear flow-ballooning instability are discussed. Theshear flow-interchange instability appears to be responsible for substormonset. The wedge wave equation is used to compute rough expressions fordispersion relations and local growth rates in the midnight region of thenightside magnetotail where the instability develops, forming the auroral beadscharacteristic of geomagnetic substorm onset. Stability analysis for the shearflow-interchange modes demonstrates that nonlinear analysis is necessary forquantitatively accurate results and determines the spatial scale on which theinstability varies.

Shear Flow‐Interchange Instability in Nightside Magnetotail as Proposed Cause of Auroral Beads as a Signature of Substorm Onset