Mesoscale thermospheric structure - current evidence and future experiments

Griffin, E.1, Aruliah, A.1, McWhirter, I.1, Yiu, I.1, Dobbin, A.1, Millward, G.1, Aylward, A.1, Charalambous, A.1, McCrea, I.2, Howells, V.2, Davis, C.2 and Kosch, M.3

1 Atmospheric Physics Lab, University College London, London, UK
2 EISCAT group, Space Science Division, Rutherford Appleton Laboratory, UK
3 SPEARS group, Department of Communication Systems, Lancaster University, UK

The experiments and model results that have revealed the extent of mesoscale thermospheric structure are presented and discussed in the context of the consequences for ion-neutral coupling studies.

The view of the upper thermosphere as a passive, slowly changing medium has evolved in recent years thanks to the combination of innovative experimentation and advances in numerical modelling. More specifically the issue of ion-neutral coupling, at all scales, has been addressed in a more fundamental manner than previously possible. The implications of the structure evident at mesoscales within the thermosphere are significant not just for the accurate representation of the thermosphere itself but also to establish a realistic view of the fully coupled magnetosphere-ionosphere-thermosphere system.

In recent years the development of advanced CCD technology has allowed an order of magnitude improvement in time resolution for the standard Fabry-Perot Interferometer measurements of upper thermosphere neutral winds and temperatures. Structure on the time scale of a few minutes has been evident in many datasets. In parallel, radar studies of ionospheric behaviour has revealed structure in ion velocities at timescales an order of magnitude smaller than the neutrals.

Combined tristatic experiments have been undertaken using the EISCAT radar and FPI measurements from Northern Scandinavia. The results have demonstrated the importance of considering the mesoscale structure in neutral winds and ion velocities when calculating accurate Joule heating rates.

Modelling studies have often underestimated neutral temperatures while overestimating neutral wind velocities in comparison to measurements. Within the self-consistent numerical models improvements have been shown when the small scale influence of electric field variations are properly considered, a possible source of the mesoscale thermospheric structure.

In order to address both the temporal and spatial aspects of the thermosphere at mesoscales a new generation of neutral measurement instrumentation, the scanning doppler imagers, have been developed and are now being deployed at several locations globally. These instruments deliver a simultaneous extended field of detailed spatial samples at relatively high time resolution and thus represent a major improvement in neutral measurement capability and also extend the scope of combined ion-neutral coupling studies.