Fig. 2

Example of simultaneous satellite-based global images and ground-based all-sky images. These images were observed at 2129 UT on December 7, 1999. a1 Polar satellite global image in the raw (CCD) coordinates with an overlaid geographical map. Auroral emission at ultraviolet 170 nm (LBHL) is shown. a2 The same satellite image as (a1) but in the magnetic coordinates (i.e., the modified APEX coordinates). b1 Ground-based all-sky image observed at Muonio (MUO, 64.6 MLAT, 105.2 MLON, \(68.02^\circ \hbox {N}\), \(23.53^\circ \hbox {E}\)) in Lapland, Finland, in the raw (CCD) coordinates. Auroral emission at 557.7 nm (green-line) is shown. b2 The same ground image as (b1) but in the geodetic coordinates (the azimuthal equidistant projection), trimmed at the elevation angle of \(5^\circ\). The dashed white lines indicate MLT and MLAT reference lines. The red line indicates the substorm onset MLT (23.2 h), from where auroral keograms were made later. The red circles in a1 and b2 indicate the field of view of the ground images for the elevation angle of \(10^\circ\), corresponding to a diameter of \(\sim 1000\hbox { km}\) (\(\sim 9^\circ\) along latitudes) to the assumed emission altitude of 110 km