Investigating High-latitude Thermosphere-Ionosphere Coupling using Data Assimilation: Current Understanding, Observational Challenges, and Outlook

The high-latitude thermosphere and ionosphere (TI) is a tightly coupled system. Their interaction occurs primarily via ion-neutral collisions. Neutrals acquire momentum through collisions with the drifting ions which significantly modifies the solar pressure-gradient driven winds and as a result the neutral winds begin to follow the drifting ions. Comprehensive results are presented from a systematic statistical study focusing on large-scale high-latitude TI coupling using 40 years of long-term thermospheric neutral wind observations from multiple ground-based stations located at various northern and southern high latitudes and four space-based instruments. First, we describe how we combine these disparate databases into a coherent picture while overcoming the sampling limitations and biases among them. The resulting statistical synopses are then compared to the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (NCAR TIEGCM, version 2.0) which encapsulates the present basic understanding of high-latitude thermosphere and ionosphere coupling. How ion-neutral coupling varies over season and solar cycle in the context of the observations and TIEGCM results is described. These results provide: 1) a comprehensive synopsis of available past and present neutral wind measurements, and 2) recommended strategies for reconciling the limitations of present theoretical models in future science question driven research projects and mission concepts.