Doppler shifted auroral Lyman-alpha spectra observed by DMSP/SSUSI

Proton auroras are produced by energetic protons that precipitate, undergo charged exchange, and subsequently emit hydrogen lines before they are ionized again. When viewed from space, the spectral shape of proton auroras is expected to shift to longer wavelengths (red shift) due to the Doppler effect. Reports on space-based observations of Doppler shift of hydrogen Lyman-alpha spectral line associated with auroral proton precipitation are still rare. The Special Sensor Ultraviolet Spectrographic Imager (SSUSI) on board the Defense Meteorological Satellite Program (DMSP) series is a line scanning imaging spectrograph covering the far ultraviolet (FUV) spectrum from ~112 to 185 nm with 160 spectral bins. Under the spectragraph mode, SSUSI is supposed to be able to provide information about proton auroral precipitation. Here we present results from our first attempt to extract Doppler shift of Lyman-alpha spectral line of Hydrogen from the SSUSI data. We show red-shifted Lyman-alpha spectra observed by SSUSI whenever the DMSP satellite traverses the auroral oval, especially during geomagnetic active time. In general, a larger red shift appears in the nightside than dayside oval crossing and in the equatorward than poleward half of the oval. These are generally consistent with in situ auroral particle observations reported previously. Thus DMSP/SSUSI provides an alternative means to monitoring the energetic of proton precipitation.