Ionospheric flow structures associated with auroral beading at substorm auroral onset

Abstract Auroral observations have shown that brightening at substorm auroral onset often consists of azimuthally propagating beads forming along a preexisting arc. However, the ionospheric flow structure related to this wavy auroral structure has been poorly understood. We investigated ionospheric flow patterns associated with auroral onset beads using line‐of‐sight flow observations from the Super Dual Auroral Radar Network (SuperDARN) and auroral images from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) ground‐based all‐sky‐imager (ASI) array. We selected events when SuperDARN radars operated in a high temporal resolution THEMIS mode (6 s) along northward looking beams, a time resolution comparable to that of the imagers, providing a unique tool to detect properties of flows associated with the substorm onset instability. We have found very fast oscillating flows (~1000 m/s) that are correlated with the onset beads propagating across the THEMIS‐mode beam meridian. Two‐dimensional radar measurements also show a wavy pattern in the azimuthal direction with a wavelength of ~78 km, which is close to the azimuthal separation of individual beads. We also used an imager and SuperDARN in Iceland and identified weak but significant azimuthal flow modulations associated with beads. These strong correlations (in time and space) between auroral beading and the fast ionospheric flows suggest that substorm onset occurs via an instability in the inner plasma sheet and is associated with intense flow shears. The flow shear is clockwise around auroral beads, consistent with converging electric fields associated with upward field‐aligned currents in the shear center. Key Points Fast oscillating flow structure associated with susbtorm auroral onset beads Clockwise flow shears associated with upward FACs observed as auroral beads Substorm onset involves formation of fast earthward/tailward flow structures