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Radar observations of auroral zone flows during a multiple-onset substorm
Authors:J P Morelli  R J Bunting  S W H Cowley  C J Farrugia  M P Freeman  E Friis-Christensen  G O L Jones  M Lester  R V Lewis  H Lühr  D Orr  M Pinnock  G D Reeves  P J S Williams  T K Yeoman
Institution:(1) Present address: Blackett Laboratory, Imperial College, London, SW7 2BZ, UK;(2) Department of Physics, University of York, York, YO1 5DD, UK;(3) NASA Goddard SFC Code 692, Greenbelt, MD 20771, USA;(4) British Antarctic Survey, Cambridge, CB3 0ET, UK;(5) Danish Meteorological Institute, DK-2100 Copenhagen, Denmark;(6) Department of Physics, University of Leicester, Leicester, LE1 7RH, UK;(7) Department of Physics, UCW Aberystwyth, Dyfed, SY23 3BZ, UK;(8) Technische Universität Braunschweig, D-38106 Braunschweig, Germany;(9) Los Alamos National Laboratory, NIS-2, MS D-436, Los Alamos, NM 87545, USA;(10) Department of Science and Technology in Education, University of Malta, Msida, Malta
Abstract:We present an analysis of ground magnetic field, ionospheric flow, geosynchronous particle, and interplanetary data during a multiple-onset substorm on 12 April 1988. Our principal results concern the modulations of the ionospheric flow which occur during the impulsive electrojet activations associated with each onset. During the first hour of the disturbance these take place every \sim12.5 min and involve the formation of a new intense westward current filament in the premidnight sector, just poleward of the preexisting extended current system driven by the large-scale flow. These filaments are \sim1 h MLT wide (\sim600 km), and initially expand poleward to a width of \sim300 km before contracting equatorward and coalescing with the preexisting current, generally leaving the latter enhanced in magnitude and/or expanded in latitude. Within the impulsive electrojets the flow is found to be suppressed to values 50–100 m s−1 or less during the first few minutes, before surging equatorward at 0.5-1.0 km s−1 during the phase of rapid coalescence. The implication is that the precipitation-induced Hall conductivity within the impulsive electrojet initially rises to exceed \sim100 mho, before decaying over a few minutes. This value compares with Hall conductivities of \sim20 mho in the quasi-steady current regions, and a few mho or less in the regions poleward of the electrojets and in the preonset ionosphere. Preliminary evidence has also been found that the flow surges propagate from midnight to the morning sector where they are associated with arrested equatorward motion or poleward contractions of the current system. These observations are discussed in terms of present theoretical paradigms of the global behaviour of fields and flows which occur during substorms.
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