Localized polar cap flow enhancement tracing using airglow patches: Statistical properties, IMF dependence, and contribution to polar cap convection

Abstract

Recent radar observations have suggested that polar cap flows are highly structured and that localized flow enhancements can lead to nightside auroral disturbances. However, knowledge of these flows is limited to available echo regions. Utilizing wide spatial coverage by an all-sky imager at Resolute Bay and simultaneous Super Dual Auroral Radar Network measurements, we statistically determined properties of such flows and their interplanetary magnetic field (IMF) dependence. We found that narrow flow enhancements are well collocated with airglow patches with substantially larger velocities (≥200 m/s) than the weak large-scale background flows. The flow azimuthal widths are similar to the patch widths. During the evolution across the polar cap, the flow directions and speeds are consistent with the patch propagation directions and speeds. These correspondences indicate that patches can optically trace localized flow enhancements reflecting the flow width, speed, and direction. Such associations were found common (~67%) in statistics, and the typical flow speed, propagation time, and width within our observation areas are 600 m/s, tens of minutes, and 200-300 km, respectively. By examining IMF dependence of the occurrence and properties of these flows, we found that they tend to be observed under B y -dominated IMF. Flow speeds are large under oscillating IMF clock angles. Localized flow enhancements are usually observed as a channel elongated in the noon-midnight meridian and directed toward premidnight (postmidnight) for +B y (ÀB y ). The potential drops across localized flow enhancements account for~10-40% of the cross polar cap potential, indicating that they significantly contribute to polar cap plasma transport.

Polar cap precursor of nightside auroral oval intensifications using polar cap arcs

“…This speed underestimates the full E × B drift because the radar echoes here were from short‐range backscatter near the radar and were thus from the E region, where flow speed is limited to below the ion acoustic speed (~400 m/s) [ Haldoupis , ; Koustov et al ., ]. The width and speed of the flow agree with those identified by using radar observations only [ Nishimura et al ., ; Zou et al ., ], indicating that they are of the same phenomena identified with different approaches.…”

“…Although plasma motion in the polar cap is traditionally viewed as a large‐scale two‐cell pattern, mesoscale flow enhancements have been identified as a common feature based on recent radar observations [ Lyons et al ., ; Nishimura et al ., ; Zou et al ., ]. These flows are elongated in the noon‐midnight direction and are narrow in the dawn‐dusk direction, having a width of a few hundred kilometers as compared to the thousands of kilometers width of large‐scale convection.…”

“…Here we employ all‐sky imagers (ASIs), which cover a wide area and provide a continuous monitoring in 2‐D. Airglow patches in an ASI have been successfully used to trace fast flow channels deep in the polar cap [ Nishimura et al ., ; Zou et al ., ]. However, their optically faint and diffuse nature does not allow for accurate determination of the flow impingement on the nightside auroral oval.…”

Localized reconnection in the magnetotail driven by lobe flow channels: Global MHD simulation

“…Recent ground‐based observations have shown that poleward boundary intensifications along the nightside auroral oval, which are considered as an ionospheric signature of magnetotail reconnection [ Zesta et al ., ], can be preceded by localized channels of enhanced flow in the polar cap. The channels have been identified in radar flow observations and optically via emissions from F region patches and auroral arcs [ Nishimura et al ., , ; Lyons et al ., ; Zou et al ., , ]. Such polar cap‐auroral connections suggest that some portion of magnetotail reconnection can be driven by localized inflows from the lobe to the plasma sheet and thus offer a possible explanation for why magnetotail reconnection and resulting plasma sheet flow channels/dipolarization fronts can be localized azimuthally.…”

Estimating along-track plasma drift speed from electron density measurements by the three Swarm satellites

“…Hence, differences between large-scale convection patterns (SuperDARN) and smaller-scale plasma drift features (Swarm), especially around plasma density irregularities (e.g. Nishimura et al, 2014;Zou et al, 2015;Yang et al, 2015), are expected to result in a reduced correspondence between the two velocity estimates. Furthermore, Yang et al (2015, Fig.…”