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Ann. Geophys., 21, 893-913, 2003
www.ann-geophys.net/21/893/2003/
© European Geosciences Union 2003
On the lifetime and extent of an auroral westward flow channel (AWFC) observed during a magnetospheric substorm
M. L. Parkinson1, M. Pinnock2, H. Ye3, M. R. Hairston4, J. C. Devlin3, P. L. Dyson1, R. J. Morris5, and P. Ponomarenko6
1Department of Physics, La Trobe University, Victoria 3086, Australia
2British Antarctic Survey, Natural Environment Research Council, Cambridge CB3 0ET, UK
3Department of Electronic Engineering, La Trobe University, Victoria 3086, Australia
4William B. Hanson Center for Space Sciences, University of Texas at Dallas, Richardson, Texas, USA
5Australian Antarctic Division, Kingston, Tasmania 7050, Australia
6Department of Physics, University of Newcastle, New South Wales 2038, Australia
Abstract. A -190-nT negative bay in
the geomagnetic X component measured at Macquarie Island ( -65° L)
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 pre-cursor 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 ~ 800 ms-1 (47 mVm-1) during the
22-min substorm onset interval 09:56 to 10:18 UT. Maximum west-ward drifts of
>1.3 km s-1 (>77 mVm-1) occurred during a ~ 5-min
velocity spike, peaking at 10:40 UT during the expansion phase. The drifts
decayed rapidly to ~ 300 ms-1 (18 mVm-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 -65° L in the evening
sector west of the Harang discontinuity. The large westward drifts were
confined to a geographic zonal channel of longitudinal ex-tent >20°
(>1.3 h magnetic local time), and latitudinal width ~2° L.
Using a half-width of ~ 100 km 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 ~ 2-h interval following the flow channel, an ~ 3° L
wide band of scatter was observed drifting slowly toward the west, with speeds
gradually decaying to ~ 50 ms-1 (3 mVm -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.
Key words. Ionosphere (auroral
ionosphere; electric fields and currents; ionosphere-magnetospehere
interactions) Magnetospheric physics (storms and substorms)
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