Global observations of proton and electron auroras in a substorm

This is the first report of a substorm observed by the IMAGE FUV instruments permitting global observations of electron and proton produced auroras. On the 28th of June 2000 at 1956 UT in the pre-substorm phase at early evening local time the proton aurora was equatorward of the electron precipitation and near midnight they were collocated. There was bright electron and proton aurora in the post midday afternoon side. The sudden brightening of the aurora at substorm onset near midnight is seen in the electrons only although there are protons present at this location. During the expansive phase both the electrons and protons expand poleward. The electron aurora forms a bright surge at the poleward boundary while the protons just show diffuse spreading. The peak intensity of the protons did not change substantially during the entire event. The proton aurora is brighter on the dusk while the electron aurora on the dawn side. As the electron surge expands poleward it leaves the protons behind. The electrons form a discrete auroral feature near the aurora-polar cap boundary, which is devoid of substantial energetic (>1 keV) proton precipitation. The presence of precipitating protons at the point where the initial brightening is seen shows that substorms are initiated on closed field lines.