Fig. 3

Polar satellite ultraviolet observations of an auroral breakup on December 7, 1999. Auroral brightness at a wavelength of 170 nm (LBHL) is shown after projection in the modified APEX magnetic coordinates at an altitude of 110 km in the polar ionosphere. The auroral brightness is converted to the corresponding energy flux of precipitating electrons that cause auroras. a Time series of full-time (36.8-s) resolution images shown in false color from left to right. The time labels of images were chosen as the center of the image accumulation period (36.8 s for the present case). An auroral breakup (red circle, 23.2 MLT, \(64.6^{\circ }\) MLAT) is first seen in the panel labeled 2128:07 UT in red. Because the previous image was taken at 2127:30 UT, we estimate that the breakup began at 2127:49 UT (i.e., the center time of the two images). b Auroral keogram sliced along the onset meridian (23.2 MLT, ± 0.2 h average). c Auroral brightness averaged over 23.0–23.4 MLT and 62–70 MLAT. The solid red line in b and c indicates the breakup at 2127:49 UT. The dashed red line indicates 2124:50 UT