Nonlinear Processes in Geophysics www.nonlin-processes-geophys.net 1023-5809 1607-7946 16 2 2009 10.5194/npg-16-365-2009 http://www.nonlin-processes-geophys.net/16/365/2009/ http://www.nonlin-processes-geophys.net/16/365/2009/npg-16-365-2009.html http://www.nonlin-processes-geophys.net/16/365/2009/npg-16-365-2009.pdf 365 372 2009-04-30 On the ionospheric coupling of auroral electric fields G. T. Marklund goran.marklund@ee.kth.se Space and Plasma Physics, School of Electrical Engineering, Royal Institute of Technology, KTH 10044 Stockholm, Sweden The quasi-static coupling of high-altitude potential structures and electric fields to the ionosphere is discussed with particular focus on the downward field-aligned current (FAC) region. Results are presented from a preliminary analysis of a selection of electric field events observed by Cluster above the acceleration region. The degree of coupling is here estimated as the ratio between the magnetic field-aligned potential drop, &Delta;&Phi;_II, as inferred from the characteristic energy of upward ion (electron) beams for the upward (downward) current region and the high-altitude perpendicular (to <b>B</b>) potential, &Delta;&Phi;_bot, as calculated by integrating the perpendicular electric field across the structure. For upward currents, the coupling can be expressed analytically, using the linear current-voltage relation, as outlined by Weimer et al. (1985). This gives a scale size dependent coupling where structures are coupled (decoupled) above (below) a critical scale size. For downward currents, the current-voltage relation is highly non-linear which complicates the understanding of how the coupling works. Results from this experimental study indicate that small-scale structures are decoupled, similar to small-scale structures in the upward current region. 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