On the lifetime and extent of an auroral westward flow channel (AWFC) observed during a magnetospheric substorm

A -190-nT negative bay in the geomagnetic X component measured at Macquarie Island (-65degreesA) showed that an ionospheric substorm occurred during 09:58 to 11:10 UT on 27 February 2000. Signatures of an auroral westward flow channel (AWFC) were observed nearly simultaneously in the backscatter power, LOS Doppler velocity, and Doppler spectral width measured using the Tasman International Geospace Environment Radar (TIGER), a Southern Hemisphere HF SuperDARN radar. Many of the characteristics of the AWFC were similar to those occurring during a polarisation jet (PJ), or subauroral ion drift (SAID) event, and suggest that it may have been a precursor to a fully developed, intense westward flow channel satisfying all of the criteria defining a PJ/SAID. A beam-swinging analysis showed that the westward drifts (poleward electric field) associated with the flow channel were very structured in time and space, but the smoothed velocities grew to similar to800ms(-1) (47 mVm(-1)) during the 22-min substorm onset interval 09:56 to 10: 18 UT Maximum westward drifts of > 1.3 km s(-1) (> 77 mV m(-1)) occurred during a similar to5-min velocity spike, peaking at 10:40 UT during the expansion phase. The drifts decayed rapidly to similar to300ms(-1) (18 mV m(-1)) during the 6-min recovery phase interval, 11:04 to 11: 10 UT. Overall, the AWFC had a lifetime of 74 min, and was located near -65degreesLambda in the evening sector west of the Harang discontinuity. The large westward drifts were confined to a geographic zonal channel of longitudinal extent > 20degrees (> 1.3 h magnetic local time), and latitudinal width similar to2degreesA. Using a half-width of similar to100km in latitude, the peak electric potential was > 7.7 kV. However, a transient velocity of > 3.1 km s(-1) with potential > 18.4 kV was observed further poleward at the end of the recovery phase. Auroral oval boundaries determined using DMSP measurements suggest the main flow channel overlapped the equatorward boundary of the diffuse auroral oval. During the similar to2-h interval following the flow channel, an similar to3degreesLambda wide band of scatter was observed drifting slowly toward the west, with speeds gradually decaying to similar to50 ms(-1) (3 mV m(-1)). The scatter was observed extending past the Harang discontinuity, and had Doppler signatures characteristic of the main ionospheric trough, implicating the flow channel in the further depletion of F-region plasma. The character of this scatter was in contrast with the character of the scatter drifting toward the east at higher latitude.

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