Scale sizes of intense auroral electric fields observed by Cluster

Abstract

Abstract. The scale sizes of intense (>0.15 V/m, mapped to the ionosphere), high-altitude (4-7 R E 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 (K p >3) are typically 2-3 km, smaller than the typical 4-5 km scale sizes during winter months and low geomagnetic activity (K p ≤3), indicating a dependence on ionospheric conductivity.

Plasma transport along discrete auroral arcs and its contribution to the ionospheric plasma convection

“…Karlsson and Marklund (1996) found that the potential variation across the E-field structure has the same order of magnitude for all events and is independent on the E-field peak scale-size. Johansson et al (2007) get similar results for monopolar events when disregarding those scale-size ranges with very large error bars (not shown in their paper is that the same relation appears when all bipolar and monopolar events are merged into one group). Even for thin field-aligned current sheets connected to discrete arcs an inverse relationship between FAC width and FAC intensity has been found Stasiewicz and Potemra, 1998).…”

“…Thus, Haerendel (2000) concluded that the overall plasma convection will not be enhanced by discrete arcs, only the energy conversion rate. However, observations by Johansson et al (2007) of a preferred occurrence of intense E-field events at density boundaries and their often non-bipolar shape indicate that rapid flows along discrete arcs (and their return currents) lead to a net plasma flux. Due to the high velocities of these flows, the net flux above arcs could play an important role for the total plasma transport from the nightside to the dayside.…”

“…Even more important for the unequal amount of plasma flux on the different sides of discrete arcs is the non-bipolar shape of the intense E-field structure. Johansson et al (2007) showed that 75% of the intense E-field peaks have a monopolar shape. A case by case inspection of our dataset reveals that, in contrast to the example shown in Fig.…”

“…In the large statistical study by Johansson et al (2007), covering all intense E-field events above 150 mV/m between 2001 and 2003, only 13% of all detected E-field peaks have magnitudes above 500 mV/m. The low number of strong E-field peaks has its reason in the high-latitude orbit of the Cluster satellites.…”

Relating field-aligned beams to inverted-V structures and visible auroras

“…Recently, 5 years of the Fast Auroral SnapshoT Explorer (FAST) data have shown that the usual width of an inverted-V varies from 20 to 40 km, which is comparable to the peak value of the typical mesoscale auroral arc width of 10-20 km (Partamies et al, 2008). From Freja and Cluster observations, the scale sizes of auroral electric field structures were reported to be ∼ 1-5 km (Karlsson and Marklund, 1996;Johansson et al, 2007). However, these dimensions are much larger than the dimensions of a typical aurora.…”