Challenges to understanding the Earth's ionosphere and thermosphere

We discuss, in a limited way, some of the challenges to advancing our understanding and description of the coupled plasma and neutral gas that make up the ionosphere and thermosphere (I-T). The I-T is strongly influenced by wave motions of the neutral atmosphere from the lower atmosphere and is coupled to the magnetosphere, which supplies energetic particle precipitation and field-aligned currents at high latitudes. The resulting plasma dynamics are associated with currents generated by solar heating and upward propagating waves, by heating from energetic particles and electromagnetic energy from the magnetosphere and by the closure of the field-aligned currents applied at high latitudes. These three contributors to the current are functions of position, magnetic activity, and other variables that must be unraveled to understand how the I-T responds to coupling from the surrounding regions of geospace. We have captured the challenges to this understanding in four major themes associated with coupling to the lower atmosphere, the generation and flow of currents within the I-T region, the coupling to the magnetosphere, and the response of the I-T region reflected in the neutral and plasma density changes. Addressing these challenges requires advances in observing the neutral density, composition, and velocity and simultaneous observations of the plasma density and motions as well as the particles and field-aligned current describing the magnetospheric energy inputs. Additionally, our modeling capability must advance to include better descriptions of the processes affecting the I-T region and incorporate coupling to below and above at smaller spatial and temporal scales.