Estimation of auroral altitude of November 9 event. (a) The magnetic field line of the MetOp3 satellite’s location (red marker) is shown by black solid line. Black dotted line is the magnetic field line at 57.3°N derived from NOAA18 satellite. The black marker is the location of the observation. The lines-of-sight of 0-, 4-, 8- and 12-degree elevation angles are shown by red, green, blue, magenta solid lines. Each estimated altitude is indicated by the corresponding dotted curve, with altitudes of 310 km, 440 km, 570 km, and 710 km. (b) The resulting auroral altitude (~490 – ~570 km) was visualized by superimposing an altitude grid on the photograph.