Increasing knowledge about atmospheric sciences, regarding both measurements and models, has highlighted the importance of the interaction between atmospheric layers. The Entire Atmosphere Global Model (EAGLE) is an important step in this direction.
The Earth’s ionosphere is a complex system of coupled dynamical, radiative and chemical processes. Ionospheric variability is mostly defined by the solar radiation flux and geomagnetic activity, but still a significant part of it (~20%) is associated with the forcing coming from the lower and middle atmosphere. The main mechanisms responsible for this connection are planetary waves, atmospheric tides, and gravity waves.
Figure 1. Components and information flow in the EAGLE model.
The proper representation of all these processes is crucial to understand the ionosphere itself and its connection to the lower layers. Historically, numerical models of the upper atmosphere layers (>80 km) and the lower atmosphere layers (<80 km) have progressed almost independently by just prescribing the lower/upper boundary conditions, which is usually a very rough approximation of all the physics happening below/above. With increasing knowledge of atmospheric sciences related to progress, both, in measurements and models, it became clear that the interrelation between atmospheric layers is important and needs to be addressed explicitly. This project is a step in this direction devoted to the development and application of the new Entire Atmosphere Global Model (EAGLE, Fig. 1) that combines models of the upper and the lower atmosphere.
To calculate the state and variability of the lower and middle atmosphere, we use the HAMMONIA model (Hamburg Model of the Neutral and Ionized Atmosphere). This model is mostly based on the 5th version of the general circulation model of the atmosphere, ECHAM5, but also contains several important additions, such as the extension of the top boundary to 250 km, inclusion of the chemical module MOZART3, and others, which allow a good representation of the lower thermosphere. The state and variability of the upper atmosphere are calculated by the GSMTIP model (Global Self-consistent Model of the Thermosphere, the Ionosphere and the Protonosphere). This model is based on the system of quasi-hydrodynamic equations of continuity, motion and heat balance for neutral and charged particles of the cold near-earth plasma in conjunction with the equation for the electric potential in the altitude range from 80 km to geocentric distance of ~15 Earth radii, thus, fully describing the ionosphere.
In order to couple both models, we programmed a coupler interface that prepares fields of both models to be transited to each other in the overlap region and allows parallel (HAMMONIA) and non-parallel (GSMTIP) codes to be simultaneously run. HAMMONIA provides neutral temperature and wind fields in the 80 – 120 km region, which is defined by gravity-wave activity, and the number densities of air, O, N, and NO at 80 km, while GSMTIP shares the results of ion-neutral interactions (Joule heating and ion drag tendencies) in the 80 – 250 km region. The new combined model already demonstrates better representation of the mean state of the thermosphere and the ionosphere than calculated by discrete models, as well as better behaviour of the ionospheric parameters under dynamically perturbed conditions of the strong sudden stratospheric warming of January 2009 as shown in Figure 1.
Figure 2. January 2009 monthly mean zonal mean temperature from 80 to 175 km. A – MIPAS observations; B – GSMTIP; C – HAMMONIA; D – EAGLE. Dashed lines mark 400 and 570 isotherms.