Wild et al. (2011) also presented an example of omega bands observed just after substorm onset in the midnight sector, dawnward of the onset region.
This is consistent with substorm onsets being very often triggered by the intrusion of plasma with lower entropy than the surrounding plasma to the onset region.
Radar observations can also be used, and can allow for direct detection of the ionospheric mapping of flow bursts to the onset region (Nishimura et al. 2014).
The experiment also revealed that the onset region of this "counter-gradient" diffusion was adjacent to the strong "gradient" diffusion region.
In "A case study in relative ULF wave timing" section, we demonstrate that each ULF wave band has significantly different onset times as a function of radial distance away from the auroral onset region.
Finally, Pi1 waves occur in minutes following substorm onset during the substorm expansion phase and auroral breakup as the aurora expands poleward and toward the dawn and dusk of the onset region.
The window length of 2 s was chosen considering that the number of samples provide sufficient amount of information about the signal, but it is short enough to assume that the seizure does not spread yet from the onset region.
Direct tests of the intrusion of new plasma to the onset region can be performed using spacecraft (Xing et al. 2010), though sparse coverage of the plasma sheet by spacecraft that makes opportunities for such a test is very limited.
CPD of channel×time×frequency EEG tensors can provide even more accurate spatial information: it was shown that the topographic map of the ictal source coincides with the clinically determined seizure onset region [11, 13].
The present observations do not have adequate spacecraft coverage near the EP onset region to determine if NENL flow activity was involved, as was inferred by Angelopoulos et al. (2010) for a different event.
