| The data of the regular low-frequency D1 E-region observations at Collm,
Germany (52°N, 15°E) in 1983–1999 are used for estimations of the intensity of short-period
perturbations of the horizontal drift velocity at 85–110 km altitude. A simple half-hourly-difference
numerical filter is used to extract perturbations with time scales of 0.7–3 h.
The average monthly standard deviations of short-period perturbations of the zonal velocity
near altitude 83 km have a main maximum in summer, a smaller maximum in winter, and
minimum values at the equinoxes. At higher altitudes the summer maximum is shifted towards
the spring months, and a second maximum of perturbation amplitudes appears in autumn at altitudes
near and above 100 km. The seasonal changes in the standard deviations of meridional velocity
show the maxima in spring and summer. A numerical model describing the propagation of a set of
harmonics modeling a spectrum of internal gravity waves in the atmosphere is used for the
interpretation of observed seasonal variations of wind perturbation intensity. Numerical
modeling reveals that the observed altitude changes in the seasonal variations of the drift
velocity standard deviations may be explained by a superposition of IGWs generated at different
levels in the troposphere and middle atmosphere. IGWs generated in the stratospheric and
mesospheric jet stream may have substantial amplitudes at altitudes near and above 100 km,
where they may modify the seasonal variations, which are typical for IGWs propagating from the
troposphere.
Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides) – Ionosphere (ionospheric irregularities) |