The role of proton precipitation in the excitation of auroral FUV emissions

Far ultraviolet remote sensing from a high-altitude satellite is extensively used to image the global aurora, derive its energetics, and follow its dynamical morphology. It is generally assumed that the observed emissions are dominated by the interaction of the precipitated electrons with the thermospheric constituents. A model to calculate far ultraviolet emissions excited by auroral electrons and protons and the secondary electrons they generate has been used to calculate the volume excitation rate of the H I Ly-alpha, O I 1304 and 1356 Angstrom, N I 1493 Angstrom multiplets, and the N-2 Lyman-Birge-Hopfield (LBH) bands. The characteristic energy and the energy flux are derived from the observed statistical distribution of precipitated protons and electrons. This model is applied to the midnight aurora, the noon cusp, and a proton-dominated aurora for moderately disturbed conditions. We show that in the first two cases, direct electron impact dominates the vertically integrated emission rate over the proton component, although proton excitation plays an important role at some altitudes in the daytime cusp. In afternoon regions of the auroral zone near the auroral boundary, secondary electrons due to proton ionization are the main source of FUV emissions. The energy dependence of the efficiency of LBH band emission viewed from high altitude is calculated for electron and proton precipitations. Maps of the N2 LBH emission excited by both components are obtained, and regions of proton-dominated auroral emission are identified. It is found that the distribution of the ratio of proton-induced to electron-induced brightness resembles maps of the ratio of the respective precipitated energy fluxes. Proton-dominated FUV emissions are thus located in a C-shaped sector extending from prenoon to midnight magnetic local times with a maximum proton contribution near the equatorward boundary of the statistical electron oval. The distribution of the Ly-alpha /LBH intensity ratio is found to mimic the ratio of the proton flux/total energy flux, although it is insufficient by itself to accurately determine the relative fraction of auroral energy carried by the protons.