# Ionospheric Science: An Example of the Importance of Diversity in Approaches to Scientific Research
## 3 Where Are We Today? Today, there are continuing strong links between ionospheric science and the applications community. These include key physics challenges (a) to understand the spatial and temporal morphology of the ionosphere and thus how this slows and refracts radio signals as they pass through the ionosphere and (b) to understand the properties, and generation of, ionospheric irregularities that can diffract and scatter radio signals, leading to unexpected variations in the signal phase and amplitude. However, we must recognise that these challenges overlap with other challenges that arise from curiosity-driven research and from links with other scientific disciplines, such as:  
1. **The role of the high-latitude ionosphere as an image of magnetospheric dynamics--and hence the value of observations of that region as a means of studying magnetospheric dynamics. The high-latitude ionosphere is strongly coupled to the magnetosphere via the magnetic field lines that thread both regions, e.g., the flow of plasma in the high-latitude ionosphere is largely driven by magnetospheric electric fields, which are in turn driven by the flow of momentum from the solar wind into the magnetosphere. These flow patterns were first recognised over sixty years ago [26], and our understanding has gradually been refined over the years as advanced ionospheric instruments have been deployed in the Arctic and Antarctic (for example, see [2; 27; 28]) and as a range of space missions, such as Cluster and THEMIS, have provided new insights into magnetosphere-ionosphere coupling (for example, see [29; 30]). However, we are still learning as there is much more to understand about the wide range of physics at work in the high-latitude ionosphere. Thus, new instruments are being developed such as the EISCAT_3D radar [31], which will enable simultaneous measurements across a large volume of the polar ionosphere. In addition, old techniques are being enhanced--such as the use of sounding rocketsto observe the small-scale plasma physics (e.g., see [32]). Note that sounding rockets are vastly superior to satellites for this purpose as they can have a low velocity (100-200 m s\({}^{-1}\)) across the magnetic field lines, and hence better resolution of plasma structures, compared to satellites, which must necessarily cross magnetic field lines much faster, typically 7 km s\({}^{-1}\);
2. **The weak coupling between the F-region ionosphere and the upper thermosphere. ** This is a critical scientific challenge as these two elements of the upper atmosphere co-exist in the same physical space, roughly 150 km to 500 km altitude. However, the thermosphere behaves as a fluid (as the mean free path of neutrals is less than a scale height), whilst the F-region ionosphere behaves as a collisionless plasma (as the ion-neutral collision frequency is less than the ion-gyrofrequency at these altitudes). The two elements are weakly coupled via ion-neutral collisions, which facilitate momentum transfers, leading to a variety of interesting effects including (a) vertical flows of F-region plasma driven by thermospheric winds at mid- and low-latitudes (for example, see [33]) and (b) equatorward thermospheric winds around local midnight at high latitudes as a result of solar-wind-driven ionospheric flows from the dayside to the nightside (for example, see [34]). Many features arising from this coupling are understood, but there is a critical underlying issue that follows from the existence of such features. Namely, we cannot fully understand the behaviour of the ionosphere without also understanding the behaviour of the thermosphere, how it is driven by flows of energy and matter, both from above (e.g., extreme ultra-violet (EUV) radiation from the Sun, electric fields and currents from the magnetosphere) and from below (e.g., gravity waves from convective activity in the troposphere, upflows of molecular species from the mesosphere and lower thermosphere).