The substorm onset instability suggested in this paper combines the features of macroscale electromagnetic disturbance of the tearing mode type and the microscale plasma turbulence, which is generated by the cross-field current instability and modulated in coherence with the large-scale disturbance. Both components are involved in a feedback loop. It is assumed that the current sheet (CS) is thin enough for cross-field current instability (CCI) turbulence to already exist. Current density variations of the large-scale disturbance result in additional CCI generation; on the other hand, that additional plasma turbulence provides in quasi-linear approximation a flux of electrons which violates the frozen-in condition for magnetized electrons and thus destabilizes the tearing mode. The process is nonlinear. However, the threshold amplitude diminishes as the CS thins, so that the instability sets in when the threshold gets smaller than the background fluctuations. ¿ 1999 American Geophysical Union