Fig. 9

IMAGE ground magnetic observations near the substorm onset longitude in Europe. a Variations in the northward (X), eastward (Y), and downward (Z) components of the magnetic field in geomagnetic coordinates. The panels are presented in order of the observatory latitude, with the top panel corresponding to the highest magnetic latitude (MLAT) station. The magnetic local time (MLT) of each observatory at 2127 UT is shown at the left of each panel. Red vertical lines indicate the times of the Akasofu initial brightening (AIB) at 2124:50 UT and the poleward expansion at 2127:50 UT, both at [23.2 MLT, 64.6 MLAT] in the all-sky images (ASIs). The five-quiet-day baseline was subtracted for each observatory. The negative bay began at 2128 UT. b Intensity of the equivalent current at an altitude of 110 km, shown along the KIR station meridian (\(103^\circ\) magnetic longitude). The equivalent current was derived by using the same data set as a, except that the van de Kamp (2013) baseline was used. c Expansion of a for weak variations beginning at 2124 UT. The average of the displayed interval was subtracted for each observatory. d Locations of some IMAGE magnetic stations overlaid on an ASI at Muonio captured during the AIB