| The thermosphere and ionosphere are two overlapping regions of the atmosphere that are tightly coupled in many ways, including chemistry, momentum transference, and heating due to velocity differences. An understanding of both the thermosphere and ionosphere is important for a number of space research and space weather applications. To examine the coupling between thermosphere and ionosphere, we developed the new 3-Dimensional Global Ionosphere-Thermosphere Model (GITM), which couples neutral and ion chemical, dynamical and energetic processes self-consistently. GITM is different than other similar models mainly in that it relaxes the hydrostatic equilibrium condition on the thermosphere and has flexible resolution.; In this thesis, GITM is used to explore responses to magnetospheric and solar forcing in the upper atmosphere and ionosphere. First, we have examined the effect of the vertical ion convection on the electron density at high-latitudes, which has been ignored in most previous studies. A vertical ion circulation in the noon-midnight meridional plane has been proposed based on our findings to explain the source of the tongue of ionization. Secondly, the impact of solar activity and geomagnetic activity on the neutral wind has been thoroughly quantified. The neutral wind dependences on solar illumination, cross polar cap potential and aurora precipitation has been separated to extract a better physical understanding of the coupled system. Finally, the thermosphere-ionosphere coupling is studied by quantifying the effects of oscillating electric fields on the Joule heating, the primary driver of global ionospheric storms. From a simple steady-state case to a multi-frequency case, a series of numerical experiments is conducted to quantify the importance of both spatial and temporal electric field variability on the Joule heating. It is found that imposing a 40 minute and 4 hour oscillation on the electric field can cause an increase in the polar averaged Joule heating by 28%. By increasing the latitudinal resolution from 5° to 1.25°, the Joule heating is increased by 20%. The impact of the vertical velocity has also been examined, although the effect in the polar cap is minimal. These can help all global models to get more realistic Joule heating and temperature. |
