High-altitude signatures of ionospheric density depletions

We present a number of Cluster passes of the auroral oval, at altitudes of around 3-4 Earth radii, where the perpendicular electric field (E) is large; several hundreds of mV/m mapped to the ionosphere, and very well correlated with the downward field-aligned current (DFAC). The multi-point nature of the Cluster measurements allows us to study the geometrical configuration and temporal properties of the associated current sheets, which are seen to be stable on time scales of at least a minute, whereas the electric field can change appreciable during this time. In situations where the ionospheric conductivity is constant, there is a correlation between E and the perpendicular magnetic field (which represents the integrated field-aligned current.) When the electric field strength instead is correlated to the local downward FAC, this indicates that it is related to an ionospheric current closure over a low-conductivity region associated with the downward current. Such density depletion have been shown in simulations to be due to the outflow of ionospheric electrons which carry the DFACs. Here we show that a simple model of the relation between the DFACs and the conductivity reproduce the presented Cluster data well.