Global Modeling of the Low and Mid-Latitude Ionospheric D and Lower E Regions and Implications for HF Radiowave Absorption

We compare D and lower E region ionospheric model calculations driven by the Whole
Atmosphere Community Climate Model (WACCM) with a selection of electron density profiles made by
sounding rockets over the past 50 years. The WACCM model, in turn, is nudged by winds and temperatures
from the Navy Operational Global Atmospheric Prediction System-Advanced Level Physics High Altitude
(NOGAPS-ALPHA). This nudging has been shown to greatly improve the representation of key neutral
constituents, such as nitric oxide (NO), that are used as inputs to the ionospheric model. We show that
with this improved representation, we greatly improve the comparison between calculated and observed
electron densities relative to older studies. At midlatitudes, for both winter and equinoctal conditions, the
model agrees well with the data. At tropical latitudes, our results confirm a previous suggestion that there is
a model deficit in the calculated electron density in the lowermost D region. We then apply the calculated
electron densities to examine the variation of HF absorption with altitude, latitude, and season and from
2008 to 2009. For low latitudes, our results agree with recent studies showing a primary peak absorption
in the lower E region with a secondary peak below 75 km. For midlatitude to high latitude, the absorption
contains a significant contribution from the middle D region where ionization of NO drives the ion
chemistry. The difference in middle- to high-latitude absorption from 2008 to 2009 is due to changes in the
NO abundance near 80 km from changes in the wintertime mesospheric residual circulation.