Evidence of Directional Filtering of Travelling Ionospheric Disturbances

IT has recently been pointed out1 that the strong thermospheric winds would act as a directional filter. In a windy thermosphere an otherwise penetrating gravity wave may become reflected or asymptotically trapped near the lower thermosphere. Because both reflexion and trapping take place below the ionospheric F2 peak, such waves may not excite travelling ionospheric disturbances of sufficient magnitude to be observed. Therefore, as far as the observation of travelling disturbances is concerned, the thermospheric wind is acting like a directional filter, that is, only those waves propagating in the proper directional channel can reach ionospheric heights and excite travelling disturbances. In the northern hemisphere the thermospheric wind locus rotates in a clockwise direction2 and, consequently, the allowed channel of propagation must also rotate in the same direction. Although experimental verification is difficult because continuous observations are required, no contradiction has been reported1.