Spatial distributions of electromagnetic field variations and injection regions during the 20 November 2007 sawtooth event

Abstract

Abstract. We report multi-spacecraft and ground-based observations of a "sawtooth" event on 20 November 2007. For this event, data from three THEMIS, two GOES, and four LANL spacecraft are available as well as those from extensively distributed ground magnetometers and all-sky imagers. In the present paper we focus on the spatial extents of the electromagnetic and particle signatures of the first "tooth". In this event, auroral images and ground magnetic bays showed two activations: a pseudo onset and a major onset (we use the term pseudo onset since the former auroral brightening did not significantly expand poleward). Ground magnetic bay observations indicate that the substorm current wedge (SCW) developed after the major onset in an azimuthally wide region of ∼14-3 h MLT. Similarly, broad magnetic bay distribution was observed also for the pseudo onset prior to the major onset. Furthermore, around the pseudo onset, magnetic dipolarisations were observed from 0.5 to 5 h MLT. These observations illustrate that, during sawtooth events, activities following not only the major onset but also the pseudo onset can extend more widely than those during usual substorms. Remarkable electromagnetic field fluctuations embedded in the dipolarisation trend were seen at 0.5 and 2.5 h MLT. In particular, comprehensive plasma and field data from THEMIS showed the presence of a long-excited weak magnetosonic wave and an impulsive large-amplitude Alfvén wave with an earthward Poynting flux at around the eastward edge of the SCW; the latter was sufficiently strong for powering aurora (140 mW/m 2 Correspondence to: S. Kasahara (kshr@stp.isas.jaxa.jp) when mapped to the ionosphere). These two activations of the electromagnetic wave were identified, corresponding to the pseudo onset and the major onset. On the other hand, the dipolarisation at geosynchronous 0 h MLT was observed only after the major onset, despite its closer location to the centre of the auroral activity in terms of the MLT; this indicates that the inner radial limit of the dipolarisation region at the pseudo onset was tailward of geosynchronous altitude at 0 h MLT. The outer radial limit of the electron injection region was also found at ∼10 R E by conjunction measurements with THEMIS satellites. These radial distributions are not significantly different to those expected for usual substorms.

The substorm current wedge in MHD simulations

“…In contrast, the reconnection simulations showed a more nonlinear effect, with the field‐aligned currents rapidly increasing as the flows were stopped in the near tail and diverted. The bubble simulations further demonstrated that the total amount of the field‐aligned current increased with increasing cross‐tail width of the depleted region, even though the flows might break up into multiple channels. This is consistent with larger substorm wedge currents in sawtooth substorms, which extend over much wider local times than individual ones [e.g., Reeves , ; Cai et al , ; Kasahara et al , ]. The flows commonly break into multiple narrow channels, separated in space and time (consistent with conclusions of Sergeev et al []), while the associated increases in B z accumulate over a wider spatial range and spread both azimuthally and radially, analogous to multiple snowplows that collect snow and dump it into one growing and expanding pile. This picture is consistent with observations that earthward flows, associated with, or preceding, a persistent dipolarization are much more localized than the dipolarizing region, even in a statistical picture [ Miyashita et al , ].…”

“…The bubble simulations further demonstrated that the total amount of the field-aligned current increased with increasing cross-tail width of the depleted region, even though the flows might break up into multiple channels. This is consistent with larger substorm wedge currents in sawtooth substorms, which extend over much wider local times than individual ones [e.g., Reeves, 1998;Cai et al, 2006;Kasahara et al, 2009].…”