Global observations of proton and electron auroras in a substorm

This is the first report of a substorm observed by the IMAGE FUV instrument s permitting global observations of electron and proton produced auroras. O n the 28(th) of June 2000 at 1956 UT in the pre-substorm phase at early eve ning local time the proton aurora was equatorward of the electron precipita tion 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 al though there are protons present at this location. During the expansive pha se both the electrons and protons expand poleward. The electron aurora form s a bright surge at the poleward boundary while the protons just show diffu se spreading. The peak intensity of the protons did not change substantiall y 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 polewar d it leaves the protons behind. The electrons form a discrete auroral featu re near the aurora-polar cap boundary, which is devoid of substantial energ etic (>1 keV) proton precipitation. The presence of precipitating protons a t the point where the initial brightening is seen shows that substorms are initiated on closed field lines.