Birkeland Current Boundary Flows Associated With Field Line Resonances

Abstract

AbstractStrong zonal flows greater than 1,000 m/s have recently been identified by Swarm in the midnight sector of the winter hemisphere auroral zone. These flows are typically observed between upward and downward field aligned current pairs and therefore are referred to as Birkeland current boundary flows (BCBFs). The Swarm observations also identify ion heating and upflows associated with the fast zonal flows. BCBFs and the resulting ion heating and upflows play an important role in thermosphere‐ionosphere‐magnetosphere coupling given their high occurrence rate in the quiet time nightside auroral zone. This paper utilizes Super Dual Auroral Radar Network (SuperDARN) high‐resolution fits to line‐of‐sight data to investigate two known BCBF events. The large (>2,000 m/s) BCBFs observed with Swarm A and B are found to be collocated with the location of field line resonances (FLRs) with periods of approximately 20–30 min. The SuperDARN high‐resolution fits determine FLR peak flow velocities of 400–700 m/s. When the FLR velocities are superimposed upon the background ionospheric convection of 400–600 m/s, this yields total east‐west velocities of 800–1,300 m/s near the time and location of the BCBF events. The peak flows of the field line resonances are shown to be bounded by pairs of upward and downward field aligned current similar to the BCBFs. The observations suggest a causal relationship between BCBFs and FLRs.