The Horizontal Wind Model (HWM) has been updated in the thermosphere with new observations and formulation changes. These new data are ground-based 630 nm Fabry-Perot Interferometer (FPI) measurements in the equatorial and polar regions, as well as cross-track winds from the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) satellite. The GOCE wind observations provide valuable wind data in the twilight regions. The ground-based FPI measurements fill latitudinal data gaps in the prior observational database. Construction of this reference model also provides the opportunity to compare these new measurements. The resulting update (HWM14) provides an improved time-dependent, observationally based, global empirical specification of the upper atmospheric general circulation patterns and migrating tides. In basic agreement with existing accepted theoretical knowledge of the thermosphere general circulation, additional calculations indicate that the empirical wind specifications are self-consistent with climatological ionosphere plasma distribution and electric field patterns.

It has long been known that the ionospheric absorption of HF radio waves is dependent on the electron density in the ionosphere. This paper examines two aspects of the absorption calculation that have not been as thoroughly investigated. First, the correct method to calculate ionospheric absorption is explored; while the Sen Wyller ray trace formulation is generally cited as the best approximation in the D and E regions of the ionosphere, the Appleton‐Hartree formulation is more consistent with the theory in the F region of the ionosphere. It is shown that either ray trace formulation can be used to calculate ionospheric absorption if the correct collision frequencies are utilized. Another frequently overlooked aspect of the attenuation calculation are the variations in the electron‐neutral and electron‐ion collision frequencies as a function of local time, season, latitude, and solar cycle. These variations result in differences on the order of 30% in the total ionospheric attenuation and should be included in absorption calculations.

We compare the two approaches that have been used to measure the lowermost ionosphere, the measurement of the propagation of very low frequency (VLF) radio waves and the in situ sampling by sounding rockets. We focus on the altitude, latitude, and zenith angle variation of the electron density profiles inferred from these two observational techniques as compared with a theoretical photochemical model. Our results show that below 68-70 km, the VLF data and the model agree better with each other than with the sounding rocket profile. At the lowest altitudes, near 60 km, both the VLF data and the model show a greater electron density at higher latitudes, consistent with a cosmic ray flux that increases with latitude, whereas the limited rocket data show a maximum at the tropics. Above 68-70 km, the VLF data and the sounding rockets agree better and at tropical latitudes, the model fails to reproduce the observations. Specifically, the calculated electron density is lower than the data by up to a factor of 2. Possible reasons for the model deficit include underestimates of the solar Lyman alpha flux, the solar X-ray flux and the mesospheric nitric oxide density. Once these three factors are mitigated, the model is in agreement with the observations between 60 and 80 km. Plain Language SummaryOver the past 50-60 years, there have been two approaches toward measuring the lowermost ionosphere (below 90 km), the D region. One is by modeling the propagation of VLF radio waves; the other is by in situ sampling via rockets. Until now, these techniques have not been rigorously intercompared. By comparing both with each other and with a theoretical photochemical model, we show how they are complementary. The very low frequency data are most accurate below 70 km, while the rocket data are more accurate above 70 km. The photochemical model can be made to agree with these data provided certain assumptions are made about the solar Lyman alpha flux, solar X-rays, and mesospheric nitric oxide (NO). Key Points: • We compare two independent techniques for measuring the D region ionosphere • Modeling VLF radio waves is most accurate below 70 km, in situ rockets are most accurate above 70 km • Photochemical model can agree with observations, but there are uncertainties concerning neutral atmosphere and solar flux inputs Supporting Information: • Supporting Information S1

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.

The Earth's upper atmosphere is a dynamic environment and neutral winds are a critical component of it. Neutral winds control a large part of the dynamics within the coupled upper atmosphere and ionosphere system and thus plays an important role in determining the state of the ionosphere-thermosphere (I-T) system at all latitudes. Due to the geometry of the Earth's geomagnetic field, neutral winds at lower latitudes can generate electric fields via the dynamo effect and push the ionospheric plasma upward and downward along the magnetic field lines (e.g.,