Abstract
Abstract. Intense high-latitude electric fields (>150 mV/m mapped to ionospheric altitude) at 4–7 RE geocentric distance have been investigated in a statistical study, using data from the Cluster satellites. The orbit of the Cluster satellites limits the data collection at these altitudes to high latitudes, including the poleward part of the auroral oval. The occurrence and distribution of the selected events have been used to characterize the intense electric fields and to investigate their dependance on parameters such as MLT, CGLat, altitude, and also Kp. Peaks in the local time distribution are found in the evening to morning sectors but also in the noon sector, corresponding to cusp events. The electric field intensities decrease with increasing latitude in the region investigated (above 60 CGLat). A dependence on geomagnetic activity is indicated since the probability of finding an event increases up to Kp=5–6. The scales sizes are in the range up to 10 km (mapped to ionospheric altitude) with a maximum around 4–5km, consistent with earlier findings at lower altitudes and Cluster event studies. The magnitudes of the electric fields are inversely proportional to the scale sizes. The type of electric field structure (convergent or divergent) is consistent with the FAC direction for a subset of events with electric field intensities in the range 500–1000 mV/m and with clear bipolar signatures. The FAC directions are also consistent with the Region 1 and NBZ current systems, the latter of which prevail only during northward IMF conditions. For scale sizes less than 2 km the majority of the events were divergent electric field structures. Both converging and diverging electric fields were found throughout the investigated altitude range (4–7 RE geocentric distance). Keywords. Magnetospheric physics (Electric fields; Auroral phenomena; Magnetosphere-ionosphere interactions)
Highlights
Converging electric fields associated with negative U-shaped potential structures, accelerating electrons downwards in the upward field-aligned current (FAC) region, have been observed by numerous satellites such as S3-3, Dynamics Explorer 1, Viking, Polar, above the primary acceleration region
Diverging electric fields associated with positive Ushaped potential structures accelerating electrons upward in the auroral return current region were first observed by the Freja and FAST satellites at low altitudes (e.g. Freja at altitudes of 1400–1770 km, Karlsson and Marklund, 1996, and FAST at altitudes of 350–4175 km, McFadden et al, 1999)
The general trend is that the small-scale FACs have directions which are consistent with the large scale Region 1 currents and, at higher latitudes, with the NBZ-current system (Iijima and Shibaji, 1987) prevailing during northward IMF conditions
Summary
Converging electric fields associated with negative U-shaped potential structures, accelerating electrons downwards in the upward field-aligned current (FAC) region, have been. Cluster observations of intense electric fields with monopolar signatures (indicating S-shaped potential structures) in the return current region at 5.0 RE geocentric distance were reported by Johansson et al (2004). Marklund et al (2004) discussed the properties of four events of intense electric fields at Cluster altitudes, the results supporting the existence of quasi-static positive U-shaped and S-shaped potential structures in the return current region. Field structures directly associated with particle acceleration map out to high altitudes These event studies have provided much insight into the temporal and spatial variations of the electric field structures. This study presents a statistical investigation of intense electric fields observed by Cluster at geocentric distances of 4–7 RE Both diverging and converging electric field structures, corresponding to positive and negative U-shaped potential structures, are found at Cluster altitudes in a way consistent with downward and upward FACs
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