We propose a high frequency electromagnetic instability of non resonant type, driven by cross current as a source for the onset of substorm expansion phase and the field line reconnection beyond 15RE. By including a normal component of the tail field, BZ, we find that the instability can be quenched provided BZ exceeds certain threshold value -1.8¿106√Te/&lgr; gauss (0.06&ggr;, for typical values in the neutral sheet).