Poleward propagating multiple arc-like forms in pulsating and diffuse auroras: Ionospheric trace of a substorm trigger?

We discuss a new kind of pre-breakup activity. This is poleward propagating multiple ark-like forms observed in diffuse or pulsating background remaining in the sky after the previous substorm. We assume these forms to be an ionospheric manifestation of the MI-ID waves propagating inside the magnetotail plasma sheet. The pre-breakup phenomenon, such as pseudobreakups and out-of-phase variations in the system of two arcs, may also be caused by propagation of these waves. Assuming the waves are responsible for the magnetic reconnection in the magnetotail, we conclude that the multiple ark-like forms in diffuse or pulsating auroras may be the ionospheric trace of a substorm trigger.