Equatorward moving auroral signatures of a flow burst observed prior to auroral onset

We present observations of a substorm that occurred on February 25, 2008. Auroral onset was observed with a multi‐spectral (λ = 427.8, 557.7 and 630.0 nm) and white light all sky imager at Gillam, Canada. An equatorward moving diffuse auroral patch was observed in the λ = 630.0 nm images at least six minutes prior to auroral onset. This form emerged from the background noise poleward of the eventual onset arc and intensified as it moved equatorward. Auroral expansion onset occurred when this form reached the onset arc location. Flows were detected by THEMIS probes P3 (TH‐D) and P4 (TH‐E) near X ∼ −11 RE nearly 90 seconds prior to auroral expansion. A small discrete arc was observed in the λ = 557.7 nm images at the westward and equatorward edge of the diffuse 630.0 nm patch nearly 2 minutes prior to expansion onset, suggesting a field‐aligned current of a substorm current wedge geometry. We conclude that the equatorward moving λ = 630.0 nm diffuse auroral patch was generated by processes associated with an earthward moving flow burst that formed prior to auroral substorm onset.