The gravity wave TID relationship: Insight via theoretical model EISCAT data comparison

Atmospheric Gravity Waves (AGWs) in the thermosphere are of particular interest because of their role in the equatorward redistribution of auroral momentum and energy input. However, their direct measurement is difficult and so they are normally traced by their ionospheric signatures, the Traveling Ionospheric Disturbances (TIDs). These can be routinely observed, especially with incoherent scatter radars like the EISCAT-facility, which measure all the fundamental ionospheric parameters. In order to reliably infer AGW parameters from TID data, however, one needs to know the physics of the AGW-TID relationship as comprehensively as possible. We investigated this relationship by means of one-to-one comparison of theoretical model results with EISCAT data for several TID events. The relevant physics, the modeling procedure and the results of the comparisons are discussed. As a representative example, one typical event is presented in some detail. We found that the AGW-TID relationship can be quantitatively understood by means of careful physical modeling. A particular simulated TID shows quantitative consistency with a particular TID in EISCAT data only for a quite specific model-AGW; thus, comprehensive AGW information can be deduced by our method. We conclude that our use of TID 'polarization information' along a single incoherent scatter beam is basically as valuable for the unique determination of a causative AGW as is traditional TID 'propagation/dispersion information'. The latter, however, requires several distributed stations. Finally, we address the possibility that radars like EISCAT could be used in future WAGS (Worldwide AGW Study) campaigns to provide almost real-time information on AGW activity for the benefit of mid-latitude monitoring stations.