Fig. 1: Evolution of auroral activity prior to substorm onset.
From: Radio emissions reveal Alfvénic activity and electron acceleration prior to substorm onset
a Ground-based all-sky camera observations of auroral evolution recorded at Muonio (geographic latitude 68.02° N, longitude 23.53° E) using a 557.7 nm filter. b, c Geomagnetic indices (AE, AU, AL) showing the corresponding magnetospheric activity. The light blue shaded interval marks the pseudo-storm period preceding the substorm. The light purple shaded region denotes the interval of wave-like auroral activity immediately preceding the substorm onset, which is identified at about 23:25:20 based on the initiation of auroral poleward expansion. Source data are provided as a Source Data file.
