Instability of the equatorial plasmasheet: a mechanism for the formation of discrete arcs

Prior to the onset of magnetospheric subtorms, a sharp transition between ''tail'' and ''dipole-like'' magnetic configuration develops. It is shown that this boundary is unstable to westward travelling waves which are fed at the expense of azimuthally drifting energetic ions in the restored dipolar configuration. Evidence will be given that the surges observed simultaneously on the ground are nothing but the projection along geomagnetic field lines of this distorted (unstable) boundary. Medium scale field-aligned current (FAC) structures will finally be shown to develop as a result of this instability process and to tend to cascade towards smaller scale structures. Electron acceleration correlate very well with these small scale structures