H Balmer lines in terrestrial aurora : Historical record and new observations by OSIRIS on Odin

• The H Balmer emissions were first identified in terrestrial aurora by Vegard (1939). The earliest photographic spectral observations are reviewed. In the subsequent decade, the intensity ratios for H alpha, H beta, and H gamma were measured, and the well-known line broadening and blue shift were established. Recently, the H alpha, H gamma, H delta, and H epsilon features have been measured by OSIRIS on Odin. The Balmer components are resolved from other auroral features using sets of synthetic spectra. The measured intensity ratios are in good agreement with an extensive set of published model calculations. The presented observations are in the polar region averaged over limb tangent altitudes from 100 to 105 km, approximately perpendicular to the terrestrial magnetic field lines, for this geometry showing Doppler broadening without obvious Doppler shifts. The OSIRIS-measured full-width at half-height of the Ha feature is 2.2 nm corresponding to an H atom velocity of 500 km s(-1) and energy approximately 1.3 keV.

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