Comment on “Substorm triggering by new plasma intrusion: THEMIS all‐sky imager observations” by Y. Nishimura et al.

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[1] In their recent paper “Substorm triggering by new plasma intrusion: THEMIS all‐sky imager observations,” Nishimura et al. [2010, hereafter N2010] describe a sequence of events leading to substorm auroral onset. They describe a distinct and repeatable sequence of substorm onset precursor events and discuss their finding in the light of different substorm onset models. They present three cases of ground‐based auroral observations in detail showing the development of a poleward boundary intensification (PBI) followed by a north‐south aligned auroral arc moving equatorward toward the latitude of the subsequent substorm onset. This development of a PBI and north‐south aurora is linked to fast plasma flows in the magnetotail resulting from enhanced reconnection near the open‐closed field line boundary. The conclusion is drawn that the earthward transport of new plasma leads to the development of a near‐ earth instability and auroral breakup about 5.5 min after the PBI formation. [2] The event studies are accompanied by a statistical analysis of more than 200 substorm onsets that were observed by All‐Sky Imagers (ASI) of the large THEMIS Ground‐Based Observatories (GBO) array [Harris et al., 2008; Mende et al., 2008]. Using the results of their statistical study the authors determine the occurrence probability of precursor auroral forms before substorm onset. They report a very impressive 84.1% occurrence frequency for the development of N‐S or E‐W auroral forms prior to substorm onset. However, here we will show that several aspects of their event selection and statistical analysis are flawed, leading to grossly exaggerated results.