Progress in investigating long-term trends in the mesosphere, thermosphere, and ionosphere
Authors: Laštovička Jan 
Published: May 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 the surface. The results of temperature trends in
the MLT region are generally consistent with older results but are developed and
detailed 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 the sign of the 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
(NLCs), whereas cooling through mesospheric shrinking is responsible for
a 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 a clear dependence on solar activity,
which results in an increasing concentration of dangerous space debris.
Significant progress was reached in long-term trends in the E-region
ionosphere, namely in foE (critical frequency of E region, corresponding to
its maximum electron density). 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 (critical frequency of F2 region, corresponding to the maximum electron density in the ionosphere)
revealed very weak but statistically significant negative trends. First
results of 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 in the Earth's magnetic field. The results of extensive modeling
reveal the dominance of secular magnetic change in trends in foF2
and its height (hmF2), total electron content, and electron temperature in the
sector of about 50∘ S–20∘ N, 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 WACCM-X (Whole Atmosphere Community Climate
Model eXtended) for changes in temperature excited by anthropogenic trace gases simultaneously
from the surface to the base of the exosphere provides results generally consistent
with observational patterns of trends. Simulation of ionospheric trends over
the whole Holocene (9455 BCE–2015) was reported for the first time.
Various problems of long-term-trend calculations are also discussed. There
are still various challenges in the 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.