Fast Auroral Snapshot observations of perpendicular DC electric field structures in downward current regions: Implications

Abstract

[1] FAST electric field data and ion drift moments are combined to allow full DC E ? (electric field perpendicular to the geomagnetic field) studies of auroral return current regions. Statistical comparison of 71 return current potential structures showed several differences between sheetlike structured perpendicular E ? field events, where the ratio of the two E ? components remains constant during the spacecraft crossing, and curved structures where the ratio varies. Sheetlike structures can be interpreted as straight arcs, but curved structures require gradients in another dimension. We define a parameter h, which is a proxy for the ratio of the potential at the spacecraft and the upgoing electron characteristic energy. Thus h is a measure of the extent to which the potential contours are closed below the spacecraft. Statistical comparison shows that U-shaped closedpotential models are mostly consistent with curved events and ionospheric effects are dominant in sheetlike structures. This result implies that the spatial structure of the events, as indicated by the ratio of the E ? components, allows us to distinguish ionospheric fields and U-shaped potentials. Statistical studies of scale sizes, magnitudes of electric fields and magnetic perturbations, and downward current density, sorted by the parameter h, reveal various interesting features. We attempt to explain these properties on the basis of different potential closure models for sheetlike and curved structures, which have important implications for models of the formation and evolution of potential structures for downward current regions.

Scale sizes of intense auroral electric fields observed by Cluster

“…The shape of the auroral structures also influences how the electric field is mapped, or, equivalently, how the potential structures are closed. Spirals and folds have been shown to be associated with non-mapping electric fields, while sheet-like structures (stable arcs), were shown to partly extend down to the ionosphere (Hwang et al, 2006). Future studies considering this subject are planned.…”

“…Another explanation would be that the electric field sometimes partly extends down to the ionosphere, giving a mismatch between the measured electric field and the up-going electrons. Examples of this have been found by Hwang et al (2006). In a study of downward electron acceleration, Newell et al (1996) found, using the DMSP satellites, that the widths of such regions had a exponential distribution with a characteristic width of 28-35 km.…”

PFISR nightside observations of naturally enhanced ion acoustic lines, and their relation to boundary auroral features

“…Further, our observations suggest that the thin dark structures could be auroral DCRs and the presence of NEIALs associated with these dark auroral features could be evidence of downward current or upgoing electron beams. The in situ studies of DCRs focused on quasi-static DCRs and they found scalesizes down to a few km (Hwang et al, 2006a).…”

Magnetosphere-ionosphere coupling during periods of extended high auroral activity: a case study

“…From a statistical study of the return current region performed by Hwang et al (2006) using data from the FAST satellite, it was shown that the detected perpendicular electric field events having a significant contribution by ionospheric fields (S-shaped potential structures coupled to the ionosphere) were mainly associated with upgoing field-aligned Poynting fluxes, whereas decoupled potential structures (Ushaped closed potential structures) showed downgoing fieldaligned Poynting fluxes.…”

“…In a recent study by Hwang et al (2006) it was shown that downgoing field-aligned Poynting fluxes from a high altitude source are mostly associated with curved auroral forms, whereas upgoing field-aligned Poynting fluxes from a low altitude source tend to be associated with straight auroral arcs (sheetlike events). The degree by which a potential structure is coupled to the low ionosphere was shown to depend on the type of structure: sheetlike events appear to have a strong contribution from ionospheric fields and are therefore a strong coupling of the potential structures to the ionosphere (low closure degree), meaning that part of the potential structure maps to the ionosphere; curved events are generated by an high altitude source and are associated with potential structures decoupled from the ionosphere (high closure degree), i.e.…”