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

[1] In their recent paper, Nishimura et al. [2012], hereafter termed N12, conclude that Pi2 magnetic field pulsations and auroral brightenings in the substorm expansion phase are driven coherently by flow bursts in the magnetotail. The authors claim not only that the flow bursts, auroral brightenings, and magnetic field perturbations all have periodicities in the Pi2 range but significantly also that the individual perturbations are all coherent and, as a consequence, infer a causal and one-to-one connection between them. In their scenario, fast earthward flows in the plasma sheet with fine structure in the Pi2 frequency range are responsible, in a piecewise and coherent fashion, for establishing one-by-one additional elements of the substorm current wedge (SCW) in response to each element of the flow burst fine structure. In other words, each subsequent flow burst adds an additional current element to the SCW, coherently driving both repeated enhancements in the aurora and step-like changes in the ground magnetic field across a wide longitudinal range such that “each Pi2 pulse starts to rise simultaneously over a wide range of latitude from the auroral zone to the magnetic dip equator as well as over a wide longitudinal range” [N12, abstract]. Each of these occurs at intervals separated by tens of seconds or minutes corresponding to the tail flow variations in the Pi2 band. The N12 claims are in stark contrast to the traditional explanation inferred from groundbased magnetic ULF wave observations that invokes bouncing Alfvén waves for the periodic structure of the Pi2s, and which necessarily includes propagation effects and precludes such large-scale coherence, based in part on the known fact that in order to establish a field-aligned current, one needs to send Alfvén waves along the field line [see, for example, Olson, 1999, and references therein]. [2] The conclusion reached in N12 that repeating flow bursts in the magnetotail directly drive auroral enhancements, related equatorward auroral streamers, and large-scale coherent Pi2 magnetic perturbation waveforms during substorm expansion phase onset relies on the predication that all auroral streamers are driven by fast flows in the magnetotail. There is, of course, evidence that some north-south aligned auroral streamers are connected to fast flows in the magnetotail [see, for example, Zesta et al., 2000]. A comprehensive examination of the mechanisms which link streamers to tail flow bursts was not addressed in N12, nor will it be addressed in this comment. Indeed, this comment is not concerned, and does not take issue, with the possible connection between tail flows and some N-S aligned arcs. However, it is the claimed coherency of the link between auroral streamers, the large-scale SCW, and global-scale coherent Pi2s which is advanced by N12whichwe challenge here; themagnetotail drivers of auroral streamers themselves are not the focus of this comment nor were they the focus of N12. [3] The proposed coherence and causal hypothesis in N12 rests upon three logical assertions. [4] 1. Expansion phase intensifications along the poleward boundary of the aurora are repetitive, and each intensification leads to an equatorward propagating auroral streamer. [5] 2. There is a one-to-one correspondence between each individual expansion phase auroral intensification and each individual magnetic pulsation. [6] 3. Each magnetic pulsation is coherent across all latitudes for the duration of the period of the formation of the substorm current wedge. [7] The observational evidence presented in N12 claims to address these three assertions, which are at the core of their hypothesis coherently linking periodic flow bursts to Pi2s waveforms, through a multi-instrument study. 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) [Mann et al., 2008] and THEMIS ground-based magnetometers [Russell et al., 2008; 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 UCLAMagnetometer Network (http://autumn.athabascau.ca), and Geophysical Institute Magnetometer Additional supporting information may be found in the online version of this article. Department of Physics, University of Alberta, Edmonton, Alberta, Canada. Now at Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, UK. Now at Department of Meteorology, University of Reading, Earley Gate, Reading, UK.