Progress in investigating long-term trends in the mesosphere, thermosphere and ionosphere
Authors: Laštovička Jan 
Published: February 2023

Abstract. This article reviews main progress in investigations of long-term trends in the mesosphere, thermosphere and ionosphere over the period 2018–2022. Overall this progress may be considered significant. The research was most active in the area of trends in the mesosphere and lower thermosphere (MLT). Contradictions on CO2 concentration trends in the MLT region have been solved; in the mesosphere trends do not differ statistically from trends near surface. The results on temperature trends in the MLT region are generally consistent with older results but develop and detailed them further. Trends in temperatures might significantly vary with local time and height in the whole height range of 30–110 km. Observational data indicate different wind trends in the MLT region up to sign of trend in different geographic regions, which is supported by model simulations. Changes in semidiurnal tide were found to differ according to altitude and latitude. Water vapor concentration was found to be the main driver of positive trends in brightness and occurrence frequency of noctilucent clouds (NLC), whereas cooling through mesospheric shrinking is responsible for slight decrease in NLC heights. The research activity in the thermosphere was substantially lower. The negative trend of thermospheric density continues without any evidence of clear dependence on solar activity, which results in increasing concentration of dangerous space debris. Significant progress was reached in long-term trends in the E-region ionosphere, namely in foE. These trends were found to depend principally on local time up to their sign; this dependence is strong at European high midlatitudes but much less pronounced at European low midlatitudes. In the ionospheric F2-region very long data series (starting at 1947) of foF2 revealed very weak but statistically significant negative trends. First results on long-term trends were reported for the topside ionosphere electron densities (near 840 km), the equatorial plasma bubbles and the polar mesospheric summer echoes. The most important driver of trends in the upper atmosphere is the increasing concentration of CO2 but other drivers also play a role. The most studied one was the effect of the secular change of the Earth’s magnetic field. The results of extensive modeling reveal the dominance of secular magnetic change in trends in foF2, hmF2, TEC and Te in the sector of about 50° S–20° N and 60° W–20° E. However, its effect is locally both positive and negative, so in the global average this effect is negligible. The first global simulation with model WACCM-X of changes of temperature excited by anthropogenic trace gases simultaneously from surface to the base of exosphere provides results generally consistent with observational pattern of trends. Simulation of ionospheric trends over the whole Holocene was reported for the first time. Various problems of long-term trend calculations are also discussed. There are still various challenges in further development of our understanding of long-term trends in the upper atmosphere. The key problem is the long-term trends in dynamics, particularly in activity of atmospheric waves, which affect all layers of the upper atmosphere. At present we only know that these trends might be regionally different, even opposite.