Figure 4. Graphs of the brightness and brightness ratio from the 630-nm [OI], 557.7-nm [OI], and 427.8-nm N2+ emissions in the substorm onset arc observed between 1130 and 1150 UT on February 1, 1990, from Poker Flat Research Range. The peak brightness was obtained by integrating the brightness between the dashed lines of Figure 3.
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