Violation of the frozen-in condition for the plasma sheet ions when the spatial scale of disturbance is of the order of the Larmor radius leads to the magnetospheric convection instability. This instability results in the separation of the convection plasma flow into jets with the spatial scale of the order of 1 km (from a few hundreds meters up to a few kilometers) at the ionosphere level. The instability develops in these regions of the plasma sheet where the contents of the hot ions in the unit magnetic flux tube vary with the distance from the Earth. This takes place at the boundaries of the plasma sheet. The instability has a large growth rate (up to 0.01 s-1). In the nighside plasma sheet region the developing structures are stretched approximately along the auroral oval. ¿ 1999 American Geophysical Union