Time-variability of the ionospheric electric field: solar wind driving and atmospheric feedback

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The Earth's atmosphere is a complex system consisting of many layers that interact in controlling its global dynamics. The ionosphere plays a key role in these dynamics, coupling the magnetic environment of geospace - 'the magnetosphere' - above to the neutral environment below. Variability in the coupling of electromagnetic fields and particles between Earth's atmosphere and space are responsible for the phenomenon of 'space weather'. This is recognised by current NERC strategy as an environmental hazard with serious global impacts, but limitations in our understanding of its effects on the atmosphere constrain our prediction capability. Ionospheric electric fields, in particular, play an important role in governing atmospheric dynamics, primarily in the thermosphere below 150 km. This is demonstrated by the UK Met Office seeking to include the thermosphere in their weather and climate model, raising the upper boundary of its Unified Model (MetUM) from 85 km to the region of 120-140 km. In order to achieve this, improved ionospheric electric field models are essential, since existing models characterise only average conditions and have no 'memory' of the prior history of variability. Capturing this variability is critical if realistic predictions of atmospheric dynamics are to be made at these altitudes. We will therefore develop the next generation model of global ionospheric electric fields that will include, for the first time, the inherent time-dependence of their morphology and driving mechanisms. Specifically, we include variability associated with three distinct sources: (i) solar wind driving (ii) dynamical processes in geospace and (iii) atmospheric feedback. Our model will be designed in collaboration with the UK Met Office, for use in their climate and space weather modelling applications such as MetUM.To achieve our goals we will utilise multi-decadal datasets of electric and magnetic field measurements from ionospheric radars and ground magnetometers, and neutral wind measurements by Fabry-Perot interferometers, to study the electrodynamics of the ionosphere, and its coupling to the neutral atmosphere. We will use upstream interplanetary spacecraft data to order our observations not only by the concurrent conditions in geospace, but by the time history of these conditions; persistent plasma and magnetic field structures, and the degree of variability. We will also use geomagnetic measurements to investigate the effects of time-variable internal magnetospheric processes. These include magnetospheric substorms, which excite convection in the ionosphere, inject energetic particles into the atmosphere, and produce the visible aurora, or northern (and southern) lights. Lastly, we will use simultaneous measurements of the electric field and neutral wind to investigate the 'flywheel' effect of the neutral wind dynamo; the ability of neutral winds to maintain the ionospheric electric field after their direct excitation subsides. Incorporating all of these time-variable effects into a new empirical model of the ionospheric electric field will provide a valuable resource for magnetospheric physics, atmospheric modellers, and space plasma physics theorists.