Ground-based optical observations of hydrogen emission in the auroral substorm

A combination of all-sky imagers (ASTV) and meridian-scanning photometers ( MSP) was used to identify the optical signature of the growth phase, onset, expansion, and recovery of 33 auroral substorms in the Alaskan sector. The discrete auroral are that brightens at auroral substorm onset was found to be poleward of the diffuse aurora that contains the H emission by a distan ce of between 10 and 300 km. The average onset time was 2215 magnetic local time (MLT) and the average geographic latitude of the onset are and the H are was 64.6 degrees and 63.6 degrees, respectively. The poleward crossover of the peak H emission occurs shortly after substorm onset, for substorm i ntensification after 2100 MLT. The peak H emission crosses back to the equa torward position during substorm recovery between 2100 and 0100 MLT. After 0100 MLT the peak H emission remains poleward of the electron-trapping boun dary for the rest of the night. During the growth phase the peak H emission moves equatorward more quickly than does the onset are and monotonically d oubles in total intensity during the equatorward motion, in a manner quite unrelated to the fluctuations in brightness of the onset are. In the onset are a reduction of 427.8-nm N-2(+) and 557.7-nm [O I] emissions just prior to onset has been reported earlier and dubbed "auroral fading". We find tha t the 630.0-nm [O I] emission rises considerably relative to the others, an d we conclude that the "fading" observed in the few minutes prior to onset is the ionospheric signature of a pulse of > 10(10) electrons cm(2) s-V-1 w ith energies less than 100 eV. Thus the observations reported here do not s upport the argument that the substorm onset begins from within the proton p recipitation, but the discovery of the soft electron pulse during "auroral fading" just prior to onset does support those substorm onset theories requ iring a pulse of Alfven speed electrons poleward of the electron trapping b oundary and in the region of upward field-aligned current.