Ionospheric ion and electron heating at the poleward boundary of a poleward expanding substorm‐disturbed region

Abstract

We present observations of a poleward propagating substorm‐disturbed region which was observed by the European Incoherent SCATter (EISCAT) radar and the Svalbard International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers in the post‐midnight sector. The expansion of the disturbance was launched by a substorm intensification which started ∼25 min after the initial onset, and ∼10 min before the disturbance arrived over Svalbard. In association with the magnetic disturbance, a poleward expanding enduring enhancement in the F region electron temperature was observed, indicative of soft electron precipitation, with a narrow band of enhanced ion temperature straddling its poleward edge, indicative of fast ion flows and ion‐neutral collisional heating. This electron temperature boundary was coincident with the poleward propagating electrojet current system detected by the high‐latitude IMAGE magnetometer stations and is taken to be a proxy for the observation of a substorm auroral bulge. The electron temperature boundary is inferred to have a width comparable or less than one radar range gate (∼60 km transverse to the magnetic field), while the region of high ion temperature was found to be approximately three gates wide, extending approximately two gates (∼120 km) poleward of the electron temperature boundary, and approximately one gate (∼60 km) equatorward. The two‐beam radar line‐of‐sight velocity data are found to be consistent with the existence of a layer of high‐speed flow in the boundary, peaking at values ∼1.5–3 km s−1, roughly consistent with the ion temperature data. The flow is directed either east or west along the boundary depending on the direction of the flow in the poleward region. We infer that the flow is deflected along and around the boundary of the substorm‐disturbed region due to the high conductivity of the latter. Variations in the flow poleward of the boundary produced no discernible magnetic effects on the ground, confirming the low conductivity of the preboundary ionosphere.