Abstract
Abstract. 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, ΔΦII, as inferred from the characteristic energy of upward ion (electron) beams for the upward (downward) current region and the high-altitude perpendicular (to B) potential, ΔΦbot, 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. There are, however, exceptions to this rule as illustrated by Cluster results of small-scale intense electric fields, correlated with downward currents, indicating a perfect coupling between the ionosphere and Cluster altitude.
Highlights
A key issue in magnetospheric and auroral physics is how electrons and ions, producing discrete aurora and contributing to planetary plasma escape, gain their energy
The events are too few to allow any firm conclusions, the results suggest that small-scale structures in the downward current region are mainly decoupled from the ionosphere, similar to smallscale structures in the upward current region
An approach to study the quasi-static coupling of high-and low altitude electric fields have been outlined here and applied to a limited set of Cluster events. The results of this preliminary study suggest that small-scale structures in the downward current region are typically decoupled from the ionosphere, similar to small-scale structures in the upward current region
Summary
A key issue in magnetospheric and auroral physics is how electrons and ions, producing discrete aurora and contributing to planetary plasma escape, gain their energy. Particle acceleration is a key issue for a vast number of phenomena in space and astrophysical plasmas It was the main scientific objective for the Viking and Freja satellite missions which made many pioneering contributions on this topic (Marklund et al, 2004). In the upward current region, negative potential structures associated with converging electric fields and an upward magnetic-fieldaligned component, are formed at altitudes between 1 and 2 RE. When passing through such potential drops, the Earthward moving electrons will increase their energy, resulting in a penetration deeper into the atmosphere and an intensification of the aurora. In the downward current region, there are two kinds of turbulence known to be operative, a current-driven ion cyclotron turbulence and an electrostatic solitary wave turbulence (Jasperse et al, 2006 a, b)
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