Contribution of proton precipitation to space-based auroral FUV observations

M. Galand and D. Lummerzheim


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 (~10%), 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
(1900Ð0400 MLT), at a magnetic latitude of 65Ð67 degrees.

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Journal of Geophysical Research, 109, A03307, doi:10.1029/2003JA010321, 2004