Abstract
Accurate photoionization rates are vital for the study and understanding
of planetary ionospheres. Previous model calculations of terrestrial
photoionization rates lack sufficient spectral resolution to account for
highly structured photoionization cross sections as well as the solar
spectral irradiance. We present new photoionization rate calculations
from CPI’s Atmospheric Ultraviolet Radiance Integrated Code [AURIC;
Strickland et al., 1999] using high-resolution (0.01 Å) solar spectra
and high-resolution (0.01 Å) atomic oxygen (O) and molecular nitrogen
(N2) photoionization cross sections. Theoretical photoionization cross
sections of O are determined utilizing the R-matrix plus pseudo-states
(RMPS) approximation whereas N2 cross sections are determined using the
R-matrix approximation. We include 34 high-resolution partial O state
photoionization cross sections and 3 high-resolution partial N2 state
photoionization cross sections with supplemental Conway [1988]
tabulations for molecular oxygen and the remaining N2 states. We find
that photoionization rates computed at 0.01 Å resolution differ
substantially from rates computed using low-resolution cross sections,
especially in the lower thermosphere below 200 km. Specifically, we find
that ionization production rate ratios exhibit variations in altitude of
more than ±40% between the high- and low-resolution cases. Past
low-resolution calculations at various low spectral resolutions do not
sufficiently account for or preserve the highly structured
auto-ionization lines in the photoionization cross sections [Meier et
al., 2007]. These features, in combination with high-resolution solar
spectra, allow photons to penetrate deeper into the Earth’s atmosphere
producing larger total ionization rates. These higher ionization rates
may finally resolve data-model discrepancies in altitude profiles of
electron densities due to the use of low-resolution photoionization
cross sections in current E-region models.