Fig. 6

Eastward component plotted over east–west cross sections of auroral images. Panels a–d indicate the measurements for events R1–R4, respectively. The horizontal yellow lines in each panel show the east and west edges of the FPI scan with an elevation angle of 45°. The vertical red lines indicate the local substorm onset times for each event, as shown by the text in the top left of each panel. For all the events, Tromsø was located at the south of the onset arc at substorm onsets. There were no all-sky camera data for ~ 10 min around 02:16 LT on 27 Feb 2009 at Tromsø