The latitudinal asymmetry of accelerated electron precipitations is analyzed using AUREOL-3 satellite data. It is found that the pattern of the precipitation of auroral electrons can be very asymmetric with the most powerful structures near the equatorial boundary of the auroral oval near midnight before substorm expansion phase onset. The presence of such a feature is interpreted as the existence of maximal magnetosphere-ionosphere decoupling in the electron precipitation structure nearest the equator. It is argued that magnetosphere-ionosphere decoupling can play the principal role in the localization of the substorm onset. The slow evolution of plasma pressure distribution under the influence of the electrostatic disturbances is analyzed. The threshold of the marginal instability is obtained. A scenario of substorm onset is suggested, which includes the development of localized dusk-dawn electric field and the formation of thin sheets of downward accelerated electrons producing rayed arc and upward accelerated ions on the equatorial edge of the large-scale electron acceleration region.