Azimuthal structure of auroral arcs associated with various disturbed magnetic conditions

We investigated the mode numbers of azimuthally spaced auroral forms in various disturbed magnetic conditions using limb-view images of ISUAL (The Imager of Sprites and Upper Atmospheric Lightnings ) on board FORMOSAT-2 satellite. We choose 557.7nm and 630.0nm as our filter wavelengths. The 557.7nm green transition line of OI(1S→1D) is mainly produced by energetic electrons with energy ≥ 1keV and the 630.0nm red-line emission of OI(1D→3P) (triplet) is mainly produced by electrons with energy ~O(100eV). The auroral arcs have bright spots with approximately equal-spaced separation, in the azimuthal direction, which allows estimate of the azimuthal mode number. The azimuthal mode number of quiet time arcs is higher (m~720) and becomes lower at moderately disturbed times (m~360). For the breakup arcs at onset, the azimuthal mode number is lower with m~220. After onset, the observed breakup arcs move explosively in poleward direction from 64° to 70° latitude in about 2 minutes. The observed azimuthal structure of the substorm breakup arc at onset is similar to the events observed by THEMIS All Sky Imagers at Fort Yukon (m~250) and is consistent with the kinetic theory of ballooning modes [Cheng, Zaharia and Gorelenkov, 2004], which showed the most unstable mode at about -8RE with azimuthal mode number m~300.