A comparison of three methods of measuring tidal oscillations in the lower thermosphere using EISCAT common programmes

The EISCAT Common Programme can be used in three ways to monitor tidal oscillations in the lower thermosphere. In Common Programme One (CPI) tristatic observations provide measurements of the ion-velocity vector at several heights in the E-region and one height in the F-region. In Common Programme Two (CP2) monostatic measurements give profiles of ion velocity in the E-region while tristatic measurements give continuous measurements of ion velocity in the F-region. From the ion velocities and the ion-neutral collision frequency, the vector of the E-region neutral wind can be determined and both east-west and north-south components of the diurnal, semi-diurnal and ter-diurnal oscillations can be identified. CP1 and CP2 also provide profiles of the field-aligned ion velocity, and these can be used to calculate the north-south component of the neutral wind without knowing the ion-neutral collision frequency, but the result is affected by any vertical component of neutral velocity. The three methods are compared and the advantages of CP2 demonstrated.