Fig. 6
From: Proton auroras during the transitional stage of substorm onset
THEMIS ASI images showing substorm onset on 2016-12-09. A few selected auroral images from ATHA ASI (lower-latitude one) and FSMI ASI (higher-latitude one) showing the substorm onset on 2016-12-09. An emission altitude of 100 km is used here based upon a triangulation technique of the arc in the overlapping area of two ASIs. A blue line denotes the FESO scan meridian. The images are indexed to facilitate referene in the text