Multiple arc-like forms in pulsating and diffuse auroras: ionospheric trace of a substorm trigger?

We report a new kind of pre-breakup activity in auroras. These are poleward propagating multiple arc-like forms observed in diffuse or pulsating background remaining in the sky after the previous substorm. We connect these forms with MHD waves excited inside the magnetosphere during sudden changes of solar wind parameters. Since the waves may be responsible for the substorm onset, the wave-associated multiple arcs in diffuse or pulsating auroras are regarded as the ionospheric trace of a substorm trigger.