Abstract
Abstract. We present a multi-instrument study of a substorm bursty bulk flow (BBF) and auroral streamer. During a substorm on 25 August 2003, which was one of a series of substorms that occurred between 00:00 and 05:00 UT, the Cluster spacecraft encountered a BBF event travelling Earthwards and duskwards with a velocity of ~500 km s−1 some nine minutes after the onset of the substorm. Coincident with this event the IMAGE spacecraft detected an auroral streamer in the substorm auroral bulge in the Southern Hemisphere near the footpoints of the Cluster spacecraft. Using FluxGate Magnetometer (FGM) data from the four Cluster spacecraft, we determine the field-aligned currents in the BBF, using the curlometer technique, to have been ~5 mA km−2. When projected into the ionosphere, these currents give ionospheric field-aligned currents of ~18 A km−2, which is comparable with previously observed ionospheric field-aligned currents associated with BBFs and auroral streamers. The observations of the BBF are consistent with the plasma "bubble" model of Chen and Wolf (1993). Furthermore, we show that the observations of the BBF are consistent with the creation of the BBF by the reconnection of open field lines Earthward of a substorm associated near-Earth neutral line.
Highlights
The current circuits of the magnetosphere-ionosphere system are a fundamental constituent of the near-Earth space environment
Previous studies have shown that the flow diversions and magnetic shears observed along the edges of bursts of high speed plasma convection in the plasma sheet, termed bursty bulk flows (BBFs), are consistent with the concept of fieldaligned currents flowing in BBFs, forming a localised current wedge (Sergeev et al, 1996; Birn and Hesse, 1996; Birn et al, 1999, 2004)
We show that the field-aligned currents detected by the Cluster spacecraft, calculated using the curlometer technique (Dunlop et al, 1988; Robert et al, 1998, and references therein), are consistent with previous ground-based studies of ionospheric currents detected in association with an auroral streamer by Amm et al (1999)
Summary
The current circuits of the magnetosphere-ionosphere system are a fundamental constituent of the near-Earth space environment. Grocott et al (2004) estimated an ionospheric field-aligned current of ∼0.2 A km−2 near the footpoint of the Cluster spacecraft as the spacecraft detected the passage of a BBF during a “quiet” period, by measuring the curl of the ionospheric flows detected by the CUTLASS radars (Lester et al, 2004) and assuming a nonsubstorm Pedersen conductivity of a few Siemens They showed that this current system was associated with auroral activity with a maximum brightness of ∼1 kR, they did not discuss the current system in terms of the Chen and Wolf (1993) model. The pitch angle distribution, energy spectra of particles, ion density and magnetic flux within the flow are shown to be consistent with the reconnection of open field lines close to the spacecraft
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