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 a uroral momentum and energy input. However, their direct measurement is difficult and so they are normally traced by their ionospheric signat ures, the Traveling Ionospheric Disturbances (TIDs). These can be rout inely observed, especially with incoherent scatter radars like the EIS CAT-facility, which measure all the fundamental ionospheric parameters . In order to reliably infer AGW parameters from TID data, however, on e needs to know the physics of the AGW-TID relationship as comprehensi vely as possible. We investigated this relationship by means of one-to -one comparison of theoretical model results with EISCAT data for seve ral TID events. The relevant physics, the modeling procedure and the r esults of the comparisons are discussed. As a representative example, one typical event is presented in some detail. We found that the AGW-T ID relationship can be quantitatively understood by means of careful p hysical modeling. A particular simulated TID shows quantitative consis tency with a particular TID in EISCAT data only for a quite specific m odel-AGW; thus, comprehensive AGW information can be deduced by our me thod. We conclude that our use of TID 'polarization information' along a single incoherent scatter beam is basically as valuable for the uni que determination of a causative AGW as is traditional TID 'propagatio n/dispersion information'. The latter, however, requires several distr ibuted 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.