Understanding the ionosphere thermosphere response to solar and magnetospheric drivers: status, challenges and open issues.

The ionosphere and thermosphere (IT) constitutes a coupled, complex and dynamical electromagnetic and photochemical system, which is sensitive to a combination of external factors: particle precipitation and electrical currents from the Earth's magnetosphere and incoming solar radiation produce dramatic effects in the IT and significantly alter its energetics, dynamics and chemistry in a way that is not well understood. This sensitivity of the IT to external factors results in large, yet often unpredictable changes in many of the variables in the IT, such as in its density, temperature, neutral and ion winds, total electron content, neutral and ion composition, electric fields, currents and conductivities. External forcing of the IT system varies over different time-scales, such as solar cycle (11-year), inter-annual (e.g. quasi-biennial), seasonal and diurnal; on top of these, geomagnetic disturbances caused by solar storms and substorms can lead to abrupt reconfigurations of the magnetospheric field-aligned and horizontal currents, setting a number of electrodynamics processes in motion. The overlapping physical and chemical phenomena occur at a range of temporal and spatial scales that are highly difficult to understand as a whole. The importance of the behaviour of this region to multiple issues related to aerospace technology, such as orbital calculations, vehicle re-entry, space debris lifetime, etc., and its potential threats to modern, technology-dependent society via geomagnetically induced currents and ionospheric scintillation of Global Navigation Satellite System signals, dictate that a more detailed understanding and accurate modelling are urgently needed. In this paper, we review the status of characterization and some of the key open issues and challenges of the IT, focusing on measurement gaps in this region as well as areas of largest discrepancies between models and data. This article is part of the theme issue 'Solar eruptions and their space weather impact'.