Doppler Shifted Auroral Hydrogen Lyman-Alpha Spectra Observed by DMSP/SSUSI

Proton auroras, when viewed from space, are subjected to Doppler red shifts. Reports on space-based observations of Doppler shift of H Lyman-alpha (Ly-alpha) spectral line associated with proton precipitation are still scarce. This study, as a first attempt, extracts Doppler shifted H Ly-alpha spectral profiles resulting from proton precipitation from data acquired with the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) on board the Defense Meteorological Satellite Program (DMSP)/F16 spacecraft. SSUSI consists of a scanning imaging spectrograph, which covers the far ultraviolet spectrum from similar to 1,120 to 1,850 angstrom, with 160 spectral bins and a spectral resolution of similar to 20 angstrom. We show redshift Ly-alpha spectra observed by SSUSI when its field of view traverses the auroral ovals. The inferred proton energy and energy flux are in close agreement with the DMSP SSJ in situ particle measurements. This study demonstrates the potential capability of SSUSI in monitoring the energetics of proton precipitation. Proton precipitation can produce hydrogen lines of emissions known as proton auroras after protons become neutralized and excited through charge exchange with atmospheric hydrogen atoms. Unlike electron auroras, proton auroras are subjected to wavelength shifts called the Doppler effect, because the light emitter is in quick motion. The Doppler shift measurements possibly allows us to estimate precipitating proton energy. When viewed from space, the Doppler effect predicts a shift toward longer wavelengths. As a first attempt, we extract the hydrogen Lyman-alpha line profile from the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) on board the Defense Meteorological Satellite Program (DMSP) F16 satellite and infer the amount of Doppler wavelength shift to the average energy of the precipitating protons. We compared the result with the SSJ particle detector on board the same DMSP satellite, and they are reasonably in good agreement although they were slightly offset between the two instruments' looking directions. This study demonstrates the potential capability of SSUSI in monitoring the energetics of proton precipitation. First report on Doppler shift Lyman-alpha emissions observed by DMSP/SSUSIThe redshift Lyman-alpha emissions are associated with auroral proton precipitationThe inferred proton energy flux and mean energy are qualitatively in agreement with particle observations