Observations of ionospheric ion flow and related convective electric fields in and near an auroral arc

Abstract

Measurements of the ionospheric ion distribution function and energetic electron and ion precipitation during an auroral substorm near local midnight are presented. Thermal ion measurements are used to determine the ion bulk flow velocity, temperature, and density. It is shown that the ion flow velocity is highly correlated with energetic electron precipitation. The convective component (perpendicular to the local magnetic field B) was found to be large (equivalent electric field E⊥ ∼ 100–200 mV/m) poleward of a series of poleward expanding arcs. Inside the arc the convective flow dropped to a low value (E⊥ ∼ 10–30 mV/m). High-velocity (∼2 km/s) ion flows parallel to B were also observed and found to be correlated with electron precipitation. Poleward of the arc the flow was directed away from the ionosphere, whereas inside the arc the flow switched to earthward. Parallel electric fields of ∼0.1 mV/m required to produce these vertical flows are shown to be consistent with energetic electron pitch angle distributions. Convective electric field measurements reported here are compared with previous observations.