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

Abstract

We examine the initial ionospheric localisation and expansion of Pil 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 Pil band. The long-period Pil pulsations at substorm onset are initially localised in longitude, and expands away from an epicentre in the ionosphere, with ∼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 Pil pulsations. The arrival of the initial long-period Pil wavepacket demonstrates the importance of global networks of ground-based magnetometers for probing substorm onset. The Pil expansion proceeds westward at a rate of approximately 1 MLT hour per ∼20 seconds, representing a very rapid expansion of the Pil signal at the ground. The resolution of the Pil localisation and the rate of expansion suggest Pil waves can play an important role in studies of the causal sequence of energy release in substorms. Copyright 2008 by the American Geophysical Union.