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

Abstract

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 ∼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 ∼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.

Using ultra-low frequency waves and their characteristics to diagnose key physics of substorm onset

“…The Meyer wavelet was used for the analysis as it closely resembles the impulsive nature of nightside ULF waves and has excellent timing resolution, and hence is ideal for defining any "onset". Milling et al (2008) found that the first ULF wave band to rise above a pre-determined noise threshold was in the long-period Pi1 wavelet band, and when the analysis was extended across all available magnetometers, the long-period Pi1 waves had a clear and coherent onset that spread out from an epicentre in the auroral zone. Murphy et al (2009a) presented an automated algorithm based on the Meyer wavelet (christened the automated wavelet estimation of substorm onset and magnetic events-AWESOME) and used it to compare the onset timing and location of long-period Pi1 waves with global auroral intensification times detailed in the Frey substorm listings (Frey et al 2004;Frey and Mende 2006).…”

“…However, Murphy et al (2009b) demonstrated that even with a more in-depth consideration of the temporal cadence of each instrument in the study, in general ULF wave onset always precedes global auroral breakup. Rae et al (2009a) studied the relationship between smaller scale auroral features observed minutes prior to auroral breakup and the onset of Pi1-2 ULF waves observed by both Milling et al (2008) and Murphy et al (2009a, b). After identifying the onset arc, Akasofu (1964) and Rae et al (2009a, b) showed that the onset of Pi1-2 ULF waves occurred at the same time and in the same location as "auroral beads" (Henderson 1994) which developed azimuthally along the onset arc.…”

“…The long-period part of Pi1/short-period part of Pi2 ULF wave band (hereafter referred to as the long-period Pi1 band or Pi1-2 for brevity) was first investigated by Milling et al (2008). In this paper, a discrete wavelet transform (DWT) based upon the Pi2 algorithm outlined by Nose et al (1998) was used to investigate the entire ULF wave spectrum during substorm expansion phase onset.…”

Comment on “Formation of substorm Pi2: A coherent response to auroral streamers and currents” by Y. Nishimura et al.

“…Since the discovery that the evolution of the intensity of the optical auroral emissions during substorm onset is strongly tied to the growth of ULF wave amplitudes, tools have been developed in order to study the spectral properties of electromagnetic wave activity [e.g., Samson and Olson, 1980;Nosé et al, 1998;Milling et al, 2008;Plaschke et al, 2008;Murphy et al, 2009;Rae et al, 2009;Kataoka et al, 2009;Rae et al, 2012]. Such analysis tools would provide a significantly better approach to characterize any potential link between Pi2 waveforms and auroral emissions than the approach used in N12.…”

“…The analysis includes observations of auroral brightness from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imagers (ASIs) [Mende et al, 2008] and magnetic field perturbations from multiple arrays of ground-based magnetometers (from Canadian Array for Realtime Investigations of Magnetic Activity (CARISMA) and THEMIS ground-based magnetometers Peticolas et al, 2008]) in addition to those from the Canadian Magnetic Observatory System (http://geomag.nrcan.gc.ca/obs/canmos-eng.php), Athabasca University THEMIS UCLA Magnetometer Network (http://autumn.athabascau.ca), and Geophysical Institute Magnetometer Array (http://www.asf.alaska.edu/program/gdc/project/magnetometer) magnetometer arrays. In this comment, we reanalyze the data sets presented in N12 and demonstrate that the ground-based magnetic field and auroral observations do not support the conclusion that repetitive fast flows in the magnetotail coherently and directly drive step-like increases in the SCW in a piecewise fashion.…”