Effects of gravity and planetary waves on the lower ionosphere as obtained from radio wave absorption measurements

This paper is an overview of effects of gravity and planetary waves on the lower ionosphere as obtained from radio wave absorption measurements. To transform gravity waves from the neutral atmosphere into the ionized component is not easy. The uncertainty caused by problems with transfer of oscillations from neutral to ionized component makes the use of daytime absorption measurements for gravity wave investigations largely impossible and allows us to use only nighttime radio wave absorption. Some results of gravity wave activity studies in the lower ionosphere, based on 5.5 year long data set, are presented: (i) The gravity wave activity is almost insensitive to QBO. (ii) It decreases from the solar cycle maximum to the solar cycle minimum by ∼30%. (iii) The Mt. Pinatubo volcanic effect is well expressed for longer periods ( > 2 hours) but not detectable at short periods (< 1 hour). (iv) There is no detectable seasonal variation at high solar activity but there is a tendency to summer maximum at medium solar activity. The planetary waves are transformed from the neutral to the ionized component adequately as for the period. It was proved that planetary wave type oscillations in the lower ionosphere are caused by the neutral atmosphere oscillations, not by solar or geomagnetic activity. Various features of planetary wave activity, inferred from ionospheric measurements, have been studied. Perhaps the most interesting results are those on long-term trends. A general increase of planetary wave activity has been observed in the 1970s and 1980s, while the level of planetary wave activity has been stable in the 1960s and 1990s.