Characteristics of quasi-static potential structures observed in the auroral return current region by Cluster
Publication date: 21 December 2004
Authors: Marklund, G. et al.
Journal: Nonlinear Processes in Geophysics
Temporal and spatial characteristics of intense quasi-static electric fields and associated electric potential structures in the return current region are discussed using Cluster observations at geocentric distances of about 5 Earth radii. Results are presented from four Cluster encounters with such acceleration structures to illustrate common as well as different features of such structures. The electric field structures are characterized by (all values are projected to 100 km altitude) peak amplitudes of ~1V/m, bipolar or unipolar profiles, perpendicular scale sizes of ~10km, occurrence at auroral plasma boundaries associated with plasma density gradients, downward field-aligned currents of ~10 microA/m², and upward electron beams with characteristic energies of a few hundred eV to a few keV. Two events illustrate the temporal evolution of bipolar, diverging electric field structures, indicative of positive U-shaped potentials increasing in magnitude from less than 1kV to a few kV on a few 100s time scale. This is also the typical formation time for ionospheric plasma cavities, which are connected to the potential structure and suggested to evolve hand-in-hand with these. In one of these events an energy decay of inverted-V ions was observed in the upward field-aligned current region prior to the acceleration potential increase in the adjacent downward current region, possibly suggesting that a potential redistribution took place between the two current branches. The other two events were characterized by intense unipolar electric fields, indicative of S-shaped potential contours and were encountered at the polar cap boundary. -- abstract truncated --Link to publication