Doppler shift of auroral Lyman α observed from a satellite

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Abstract
The first documentation of Doppler shifted auroral Lyman α emission resulting from incident energetic protons in the auroral regions has been made using nadir VUV satellite spectral observations. The auroral Lyman α emission from high‐velocity protons is expected to show a red shifted wavelength displacement based on ground‐based observations of Balmer lines. VUV spectra (1100–1900Å) taken over five sample auroral oval crossings by a nadir‐viewing satellite in 1978 consistently show the Lyman α emission displaced toward longer wavelengths with a larger line width. The intensity peaks were shifted up to 4 Å when the geocoronal Lyman α emission profile was subtracted from the Lyman α emission profile observed over the auroral regions. The optical observations infer the auroral proton precipitation with average energies of 34 keV and an energy flux of 0.1 erg cm−1s−1 sr−1 when interpreted according to available model calculations. These values agree reasonably well with the average values for the characteristics of nightside incident auroral protons based on previous statistical satellite particle precipitation observations.