Abstract
Measurements of ionospheric absorption made at four neighbouring European stations are used to determine horizontal phase trace velocities of plane-wave modes which are used to model spatial phase shifts of quasiperiodic absorption variations in the lower E-region. A period of low solar activity (IQSY) is analysed. Prevailing directions of motions are found to be in the N-E and the S-W sector. The phase trace speeds range between a few tens and a few hundreds of km/d. Travelling disturbances appear to exist at lower E-region heights not only during the winter but also during the summer. Whereas wavelike perturbations in the E-region during the winter may be attributed to Rossby waves propagating upward from the lower atmosphere, the appearance of travelling disturbances in the summer ionosphere, which is normally shielded from this type of wave by the stratospheric-mesospheric easterlies, points towards additional generation mechanisms. Possibly the lower thermosphere itself acts as a source of planetary-scale waves. Another mechanism might be perturbations due to external modes similar to the five-day wavenumber 1 oscillation in the middle atmosphere.
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