Thermospheric ionization from auroral particle precipitation observed by
the SSUSI satellite instruments
Abstract
Solar, auroral, and radiation belt electrons enter the atmosphere at
polar regions leading to ionization and affecting its chemistry. Climate
models usually parametrize this ionization and the related changes in
chemistry based on satellite particle measurements. Precise measurements
of the particle and energy influx into the upper atmosphere are
difficult because they vary substantially in location and time. Widely
used particle data are derived from the POES and GOES satellite
measurements which provide electron and proton spectra. We present the
electron energy and flux measurements from the Special Sensor
Ultraviolet Spectrographic Imager (SSUSI) instruments on board the
Defense Meteorological Satellite Program (DMSP) satellites. This
formation of now three operating satellites observes the auroral zone in
the UV from which electron energies and fluxes are inferred in the range
from 2 keV to 20 keV. We use these observed electron energies and fluxes
to calculate ionization rates and electron densities in the upper
mesosphere and lower thermosphere (≈ 80–200 km). We present our
validation study of the SSUSI-derived electron densities to those
measured by the ground-based EISCAT radar stations. We find that with
the current standard parametrizations, the SSUSI-derived auroral
electron densities (90–150 km) agree well with EISCAT measurements,
with differences between +/- 20% for F18, and +/- 50 % for F17. The
largest differences are at the lower end of the altitude range because
there the electron densities decline very rapidly.