Figure 2. Geographic latitude of the intensity maxima for three auroral arcs from the Poker Flat Research Range meridian-scanning photometer record for February 1, 1990. The 557.7-nm [OI] emission was used for the electron-induced arcs, and the 486.1-nm Hbeta emission was used for the H arc. The location of the arcs was obtained graphically, with a curved Earth, and no allowance was made for atmospheric scattering or extinction. The assumed altitude in all three cases was 110 km. Note that if a higher altitude were used for the less energetic electron onset arc, the separation of the arcs would be even more apparent.
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