Distinction between auroral substorm onset and traditional ground magnetic onset signatures

To evaluate whether ground magnetic responses traditionally viewed as signatures of substorm onset may be related to postonset auroral streamers rather than to substorm onset seen in aurora, we have analyzed ground magnetic and all‐sky image responses for 14 substorm onsets selected solely on imager coverage. We find evidence supporting this possibility, the response being approximately coincident for midlatitude positive bay and Pi2 pulsation enhancements and for abrupt auroral zone H decreases at stations near a streamer. These are all signatures of current wedge formation, indicating that substorm current wedge formation may be more of a response to plasma sheet flow channels associated with postonset streamers than to the process leading to auroral onset and that the current wedge may develop via a series of distinctly narrower wedge‐like structures. We also find evidence that periods of streamers can occur more than once during a substorm expansion phase and can give magnetic signatures of multiple onsets even if there are no additional auroral onsets. Furthermore, the peak auroral zone H decreases are seen in association with streamers and at times varying from just a few minutes to well over a half hour after substorm auroral onset if there is a prolonged period of streamers. This indicates that peak substorm AE and AL values are likely related to features of postonset streamers and their locations relative to observing stations and are not a measure of the strength of the substorm auroral onset processes. We discuss possible additional implications on substorm expansion phase development.