Global modeling of the low‐ and middle‐latitude ionospheric D and lower E regions and implications for HF radio wave absorption

Abstract

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.

Characterization of Short‐Wave Fadeout Seen in Daytime SuperDARN Ground Scatter Observations

“…The Doppler flash feature can also be analyzed as arising from a change in conductivity of the ionosphere, which leads to a change in phase path of the EM wave. Another study done by Siskind et al (2017) showed that ionospheric absorption peaks at the lower E region and D region of the ionosphere where the ionization maximizes due to enhancement of X-ray and EUV flux. Clearly, the Doppler flash and D and E region absorption phenomenon are related and have similar driving processes.…”

An Intercomparison of VLF and Sounding Rocket Techniques for Measuring the Daytime D Region Ionosphere: Theoretical Implications

“…Same as Figure but for midlatitude sounding rocket data taken from Wallops Island, VA. The rocket data are the five profiles listed in Table 1 of Siskind et al (). The VLF data (solid red up to the top of the waveguide; dashed extrapolation above) is taken from the height and steepness values given in Figure 6 of Thomson et al () for a solar zenith angle of 45° (i.e., β = 0.315 km −1 and km).…”

“…Here we simply note that both the VLF community and the sounding rocket community are interested in the same atmospheric phenomena, which affect the D region. These can include solar variability (Danilov, ; Macotela et al, ; Mechtly, Bowhill, et al, ; Thomson et al, ), planetary waves (related to the winter anomaly discussed in the sounding rocket community; Beynon et al, ; Friedrich et al, ; Garcia et al, ; Pal et al, ; Siskind et al, ; Smith et al, ), and solar perturbations such as flares and eclipses (Bouderba et al, ; Chakraborty et al, ; Clilverd et al, ; Mechtly, Sechrist, et al, ; Mitra & Rowe, ; Singh et al, ). However, despite decades of work measuring the lower ionosphere by these techniques, with only a few exceptions, they typically are not intercompared.…”

“…Here we expand upon these initial comparisons to include theoretical calculations of the D region electron density profile as presented by Siskind, Martin, et al () and Siskind et al (; hereinafter referred to as S15 and S17) and the newer VLF data referred to above. We will also compare the HF (high frequency, 3‐30 MHz) absorption predicted by the electron densities inferred from the VLF reflection technique with previously published absorption calculations that relied upon rocket data.…”

Ionospheric Detection of Explosive Events

“…The variability in the ionosphere due to forcing from the stratosphere during a sudden stratospheric warming was more accurately reproduced by inclusion of realistic upward propagating GWs. Other examples of WACCM coupling to other models are described by Siskind et al () and Pedatella et al (). Further developments along these lines are being carried out, including assimilation of low‐altitude weather patterns, and validation with observations at low latitudes (McDonald et al, ).…”