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

Far ultraviolet remote sensing from a high-altitude satellite is extensivel y used to image the global aurora, derive its energetics, and follow its dy namical morphology. It is generally assumed that the observed emissions are dominated by the interaction of the precipitated electrons with the thermo spheric constituents. A model to calculate far ultraviolet emissions excite d by auroral electrons and protons and the secondary electrons they generat e 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 Lym an-Birge-Hopfield (LBH) bands. The characteristic energy and the energy flu x are derived from the observed statistical distribution of precipitated pr otons and electrons. This model is applied to the midnight aurora, the noon cusp, and a proton-dominated aurora for moderately disturbed conditions. W e show that in the first two cases, direct electron impact dominates the ve rtically integrated emission rate over the proton component, although proto n excitation plays an important role at some altitudes in the daytime cusp. In afternoon regions of the auroral zone near the auroral boundary, second ary 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 o f proton-dominated auroral emission are identified. It is found that the di stribution of the ratio of proton-induced to electron-induced brightness re sembles maps of the ratio of the respective precipitated energy fluxes. Pro ton-dominated FUV emissions are thus located in a C-shaped sector extending from prenoon to midnight magnetic local times with a maximum proton contri bution near the equatorward boundary of the statistical electron oval. The distribution of the Ly-alpha /LBH intensity ratio is found to mimic the rat io of the proton flux/total energy flux, although it is insufficient by its elf to accurately determine the relative fraction of auroral energy carried by the protons.