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

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Abstract
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 arc that brightens at auroral substorm onset was found to be poleward of the diffuse aurora that contains the H emission by a distance of between 10 and 300 km. The average onset time was 2215 magnetic local time (MLT) and the average geographic latitude of the onset arc and the H arc was 64.6° and 63.6°, respectively. The poleward crossover of the peak H emission occurs shortly after substorm onset, for substorm intensification after 2100 MLT. The peak H emission crosses back to the equatorward position during substorm recovery between 2100 and 0100 MLT. After 0100 MLT the peak H emission remains poleward of the electron‐trapping boundary for the rest of the night. During the growth phase the peak H emission moves equatorward more quickly than does the onset arc and monotonically doubles in total intensity during the equatorward motion, in a manner quite unrelated to the fluctuations in brightness of the onset arc. In the onset arc a reduction of 427.8‐nm N2+ and 557.7‐nm [O I] emissions just prior to onset has been reported earlier and dubbed “auroral fading”. We find that the 630.0‐nm [O I] emission rises considerably relative to the others, and we conclude that the “fading” observed in the few minutes prior to onset is the ionospheric signature of a pulse of > 1010 electrons cm2s‐V−1 with energies less than 100 eV. Thus the observations reported here do not support the argument that the substorm onset begins from within the proton precipitation, but the discovery of the soft electron pulse during “auroral fading” just prior to onset does support those substorm onset theories requiring a pulse of Alfven speed electrons poleward of the electron trapping boundary and in the region of upward field‐aligned current.