An overview of long-term trends in the lower ionosphere below 120 km

The increasing concentration of greenhouse gases in the atmosphere is expected also to modify the mesosphere and lower thermosphere (MLT region). However, the greenhouse cooling instead of heating-at these heights is revealed by models and generally confirmed by observations. This should more or less affect various ionospheric parameters at these heights. The spatial and temporal structure of temperature trends in the MLT region is quite complex and, therefore, such structure should occur for trends in the lower ionosphere as well. In the lower part of the ionosphere below about 90 km, the rocket measurements of electron density, the indirect phase reflection height measurements and the A3 radio wave absorption measurements reveal trends corresponding to cooling and shrinking of the mesosphere, while riometric measurements of cosmic noise absorption provide inconclusive results. The radio wave absorption and rocket electron density measurements clearly display a substantial dependence of trends on height. Ionosonde data show that there is a model-expected trend in the maximum electron concentration of the E region ionosphere; foE is slightly increasing. On the other hand, the height of the normal E layer, h'E, is slightly decreasing. The nighttime LF radio wave reflection height measurements near 95 km support an idea of increasing electron density. However, rather scarce rocket measurements display a negative trend in electron density at 90-120 km. The role of the solar cycle and other longer-term variability of natural origin in the determination of observational trends must not be neglected. In spite of the general qualitative agreement with model expectations, there is still some controversy between various observational trend results (hopefully, apparent rather than real), which needs to be clarified.