Bibliography

Seasonal variability in global eddy diffusion and the effect on neutral density

We describe a method for making single-satellite estimates of the seasonal variability in global-average eddy diffusion coefficients. Eddy diffusion values as a function of time were estimated from residuals of neutral density measurements made by the Challenging Minisatellite Payload (CHAMP) and simulations made using the thermosphere-ionosphere-mesosphere electrodynamics global circulation model (TIME-GCM). The eddy diffusion coefficient results are quantitatively consistent with previous estimates based on satellite drag observations and are qualitatively consistent with other measurement methods such as sodium lidar observations and eddy diffusivity models. Eddy diffusion coefficient values estimated between January 2004 and January 2008 were then used to generate new TIME-GCM results. Based on these results, the root-mean-square sum for the TIME-GCM model is reduced by an average of 5\% when compared to density data from a variety of satellites, indicating that the fidelity of global density modeling can be improved by using data from a single satellite like CHAMP. This approach also demonstrates that eddy diffusion could be estimated in near real-time from satellite observations and used to drive a global circulation model like TIME-GCM. Although the use of global values improves modeled neutral densities, there are limitations to this method, which are discussed, including that the latitude dependence of the seasonal neutral-density signal is not completely captured by a global variation of eddy diffusion coefficients. This demonstrates the need for a latitude-dependent specification of eddy diffusion which is also consistent with diffusion observations made by other techniques.

Pilinski, M.; Crowley, G.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 04/2015

YEAR: 2015     DOI: 10.1002/2015JA021084

annual; eddy diffusion; neutral density; satellite drag; seasonal variability; semiannual

Annual/semiannual variation of the ionosphere

We investigated the relationship between the systematic annual and semiannual variations in the ionosphere and thermosphere using a combination of data analysis and model simulation. A climatology of daytime peak density and height of the ionospheric F₂ layer was obtained from GPS radio occultation measurements by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) during 2007\textendash2010. These measurements were compared to simulations by the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM). Model reproduction of the ionospheric annual and semiannual variations was significantly improved by imposing seasonal variation of eddy diffusion at the lower boundary, which also improves agreement with thermospheric density measurements. Since changes in turbulent mixing affect both the thermosphere and ionosphere by altering the proportion of atomic and molecular gases, these results support the proposition that composition change drives the annual/semiannual variation in both the neutral and ionized components of the coupled system.

Qian, Liying; Burns, Alan; Solomon, Stanley; Wang, Wenbin;

Published by: Geophysical Research Letters      Published on: 05/2014

YEAR: 2013     DOI: 10.1002/grl.50448

annual/semiannual variations; climatology; eddy diffusion; gravity waves; neutral density and composition; NmF2

Seasonal variation of thermospheric density and composition

Thermospheric neutral density and composition exhibit a strong seasonal variation, with maxima near the equinoxes, a primary minimum during northern hemisphere summer, and a secondary minimum during southern hemisphere summer. This pattern of variation is described by thermospheric empirical models. However, the mechanisms are not well understood. The annual insolation variation due to the Sun-Earth distance can cause an annual variation, large-scale interhemispheric circulation can cause a global semiannual variation, and geomagnetic activity can also have a small contribution to the semiannual amplitude. However, simulations by the National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) indicates that these seasonal effects do not fully account for the observed annual/semiannual amplitude, primarily because of the lack of a minimum during northern hemisphere summer. A candidate for causing this variation is a change in composition, driven by eddy mixing in the mesopause region. Other observations and model studies suggest that eddy diffusion in the mesopause region has a strong seasonal variation, with eddy diffusion larger during solstices than equinoxes, and stronger turbulence in summer than in winter. A seasonal variation of eddy diffusion compatible with this description is obtained. Simulations show that when this function is imposed at the lower boundary of the TIE-GCM, neutral density variation consistent with satellite drag data and O/N₂ consistent with measurements by TIMED/GUVI, are obtained. These model-data comparisons and analyses indicate that turbulent mixing originated from the lower atmosphere may contribute to seasonal variation in the thermosphere, particularly the asymmetry between solstices that cannot be explained by other mechanisms.

Qian, Liying; Solomon, Stanley; Kane, Timothy;

Published by: Journal of Geophysical Research      Published on: 01/2009

YEAR: 2009     DOI: 10.1029/2008JA013643

eddy diffusion; thermospheric annual/semiannual variation; thermospheric density and composition