Abstract
Abstract. The scale sizes of intense (>0.15 V/m, mapped to the ionosphere), high-altitude (4–7 RE geocentric distance) auroral electric fields (measured by the Cluster EFW instrument) have been determined in a statistical study. Monopolar and bipolar electric fields, and converging and diverging events, are separated. The relations between the scale size, the intensity and the potential variation are investigated. The electric field scale sizes are further compared with the scale sizes and widths of the associated field-aligned currents (FACs). The influence of, or relation between, other parameters (proton gyroradius, plasma density gradients, and geomagnetic activity), and the electric field scale sizes are considered. The median scale sizes of these auroral electric field structures are found to be similar to the median scale sizes of the associated FACs and the density gradients (all in the range 4.2–4.9 km) but not to the median proton gyroradius or the proton inertial scale length at these times and locations (22–30 km). (The scales are mapped to the ionospheric altitude for reference.) The electric field scale sizes during summer months and high geomagnetic activity (Kp>3) are typically 2–3 km, smaller than the typical 4–5 km scale sizes during winter months and low geomagnetic activity (Kp≤3), indicating a dependence on ionospheric conductivity.
Highlights
Intense quasi-static auroral electric fields have been observed by Cluster in both the upward current region (Vaivads et al, 2003; Figueiredo et al, 2005) and the downward current region (Marklund et al, 2001; Johansson et al, 2004)
The associated potential structures can be either S-shaped or Ushaped, corresponding to monopolar and bipolar electric field signatures, respectively. Both types have been observed and the S-shaped potential structures have been shown to be associated with the sharp plasma density gradient at the plasma sheet boundary, while U-shaped potential structures occur inside the plasma sheet at less distinct gradients (Johansson et al, 2006; Marklund et al, 2007)
Temerin and Carlson (1998) discussed the current-voltage relation in the downward field-aligned currents (FACs) region and presented a model based on average charge neutrality but with the possibility of contribution to the parallel electric field from anomalous resistivity
Summary
Intense quasi-static auroral electric fields have been observed by Cluster in both the upward current region (Vaivads et al, 2003; Figueiredo et al, 2005) and the downward current region (Marklund et al, 2001; Johansson et al, 2004). The associated potential structures can be either S-shaped or Ushaped, corresponding to monopolar and bipolar electric field signatures, respectively Both types have been observed and the S-shaped potential structures have been shown to be associated with the sharp plasma density gradient at the plasma sheet boundary, while U-shaped potential structures occur inside the plasma sheet at less distinct gradients (Johansson et al, 2006; Marklund et al, 2007). These electric fields have a component parallel to the magnetic field, accelerating particles upward and downward. Theories attempting to explain parallel the electric fields include, e.g., strong double layers (Block, 1972), weak double layers (Temerin et al, 1982), Alfven waves (Song and Lysak, 2001), anomalous resistivity (Hudson and Mozer, 1978) and magnetic mirror supported fields (Knight, 1973; Chiu and Schulz, 1978). Temerin and Carlson (1998) discussed the current-voltage relation in the downward FAC region and presented a model based on average charge neutrality but with the possibility of contribution to the parallel electric field from anomalous resistivity. Jasperse (1998) described downward parallel electric fields in a kinetic model, and Jasperse and Grossbard (2000) developed an analogue to the Alfven-Falthammar formula for the downward current region
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