Spontaneous disruptions of magnetotail energy are supposed to be caused by a tearing mode instability. We show how the tail current sheet becomes unstable in the presence of a sufficiently large magnetic shear. The instability occurs because the magnetic shear removes the stabilizing influence of the gyroscopic electrons. Ion Landau damping determines the growth rate of the instability. A violation of the magnetic line conservation in a narrow region due to finite electron inertia would allow three-dimensional reconnection and flux rope formation. The resulting flux ropes are smaller in size and cause, therefore, weaker and more localized perturbations of the magnetosphere. ¿ American Geophysical Union 1991