Contribution of proton precipitation to space-based auroral FUV observations

Imaging from space offers a unique way to access the global picture, and its temporal variability, of the particle energy input over the auroral ovals. Electron characteristics are inferred from the analysis of auroral images taken from space in two different spectral bands in UV or visible. Usually, only the electron component of the precipitation is considered, as most of the particle energy is carried by electrons. However, at some locations and certain times protons are a major energy source, that is, a major ionization and excitation source of the atmosphere. The response of POLAR/UVI, IMAGE/WIC and SI13, and TIMED/GUVI (used for retrieving the electron components) to proton precipitation is estimated. Secondary electrons produced within the proton beam also contribute to auroral emissions. Since they are less energetic than the secondary electrons produced in electron aurora, they have a different spectral signature. In addition, for a given energy flux, protons are usually more efficient at ionizing than electrons and yield larger values of the Pedersen ionospheric conductance. Therefore the difference between proton and electron aurora can lead to misinterpretation when brightness ratios are used to derive ionospheric conductances with parameterizations that are based on electron aurora. The validation and limitations of auroral analysis are discussed, especially at the equatorward edge of the afternoon oval, where protons are a significant energy source. In regions of >4 keV electron precipitation, the presence of proton precipitation, even modest (similar to10%), yields a large underestimation of both the electron mean energy and the energy flux. Overall, the presence of proton precipitation yields a poor estimation of the electron mean energy. In proton-dominated aurora, the Pedersen and Hall conductances are always underestimated with a large discrepancy for POLAR/UVI. However, in location where the protons are not dominant and the electron precipitation is not too hard, it is legitimate to estimate the particle characteristics and ionospheric conductances from the FUV brightnesses assuming pure electron precipitation. This is true in particular for the period around midnight (1900-0400 MLT), at a magnetic latitude of 65-67degrees.