Ionospheric localisation and expansion of long-period Pi1 pulsations at substorm onset

We examine the initial ionospheric localisation and expansion of Pi1 pulsations associated with a substorm onset observed on 1st November 2006 with the combined CARISMA and THEMIS GMAG network of ground-based magnetometers. We demonstrate how the first ionospheric pulsation disturbance lies in the long-period Pi1 band. The long-period Pi1 pulsations at substorm onset are initially localised in longitude, and expands away from an epicentre in the ionosphere, with similar to 16 s timing between stations. We further establish a link between the location of the downward field-aligned current (FAC) element which subsequently develops within the substorm current wedge (SCW), and the initial location of the onset of long-period Pi1 pulsations. The arrival of the initial long-period Pi1 wavepacket demonstrates the importance of global networks of ground-based magnetometers for probing substorm onset. The Pi1 expansion proceeds westward at a rate of approximately 1 MLT hour per similar to 20 seconds, representing a very rapid expansion of the Pi1 signal at the ground. The resolution of the Pi1 localisation and the rate of expansion suggest Pi1 waves can play an important role in studies of the causal sequence of energy release in substorms.