Fig. 4From: Proton auroras during the transitional stage of substorm onsetTHEMIS ASI images showing substorm onset on 2015-03-18. A few selected THEMIS ASI images showing the substorm onset on 2015-03-18. The format is the same as Fig. 1, except that the blue line here denotes the FESO scan meridian. The images are indexed to facilitate referene in the textBack to article page