Association of Equatorward Extending Streamers with Ground Magnetic Perturbations and Geosynchronous Injections

It is established that auroral streamers are associated with ground magnetic perturbations at auroral latitudes. Here we present a detailed investigation on the association of equatorward extending auroral streamers with ground magnetic perturbations equatorward of the auroral oval using THEMIS (Time History of Events and Macroscale Interactions) imagers and ground magnetometers. Based on the analysis of four events, including both substorm and nonsubstorm events, we show that the ground magnetic perturbations at subauroral latitudes were not seen prior to the streamer onset and the peak negative bay at subauroral latitudes coincided with the equatorward extending auroral streamers. Although the stations close to the poleward boundary of auroral oval showed modest H-component perturbations in response to the substorm onset and poleward boundary intensifications (PBIs), the abrupt and large responses were associated with the onset of streamers. The effect of streamers is also observed in the AL index in terms of abrupt negative deflections, indicating that the AL index may not always represent the strength of the substorm auroral onset processes. Results confirm that ground magnetic field perturbations that are traditionally perceived as signatures of substorm onset are indeed related to auroral streamers. These results are further supported by the remarkable correlation of positive bay onset and sudden Pi2 enhancement at mid and low latitude with the onset of auroral streamers irrespective of the classical substorm onset. In addition, the magnitude of positive bay and the amplitude of Pi2 pulsations particularly at subauroral latitudes enhanced during the events of equatorward extending streamers. Further, we show that, in general, all streamer events were accompanied by dipolarization and electron flux enhancement at geosynchronous orbit that continue to strengthen during the emergence of groups of streamers.