Plate 1. Meridian-scanning photometer (MSP) plots of auroral emissions as a function of elevation angle from north to south through Z and UT showing the growth phase and onset for a substorm on February 1, 1990, at Poker Flat Research Range. The area between the two vertical lines on the H emission panel is where the H emission shows increased equatorward motion. The black arrow indicates the location of the peak H emission at onset (140° elevation from north), and the blue arrows indicate the location of the electron-induced arc at onset (120° elevation from north).

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