LT-TIME
Working
Group on
Long-Term
Trends in the Mesosphere, Thermosphere and Ionosphere
(A
joint venture of IAGA /ICMA)
SCIENTIFIC OBJECTIVES
The WG joins scientists interested in studies the ionosphere, thermosphere
and mesosphere long-term changes and trends. Such changes are observable
in various parameters, but their interpretation is often ambiguous, since
at altitudes studied there are strong long-term changes of solar origin,
as those induced by the eleven-year solar cycle and longer solar variations
and/or changes. Moreover, some long-term changes of unknown origin seem
to have occurred. In some parameters in some layers (e.g., foF2), the changes
may have macro-regional character with even opposite signs. Some results
are apparently rather contradictory. These considerations have resulted
in five main objectives of the WG in studying the ionosphere, thermosphere
and mesosphere:
-
To identify as early as possible which of the many observed properties
should be selected for further studies. This is the very short-term objective.
-
To prepare a scientific assessment of long-term changes and trends. This
is the short-term objective.
-
To broaden and deepen observational knowledge as well as scenario/model
understanding of long-term changes and trends.
-
To promote discussion and consensus on the above issues by holding international
workshops and symposia.
-
To distinguish between natural and anthropogenic changes and trends and
to establish unambiguously the magnitude and role of changes and trends
of man-made origin. This is the long-term objective/perspective, which
cannot be reached within a few years, of course.
The trend in the stratosphere is relatively better understood. Results
indicate good consistency between model-simulated and observed trends in
most cases. In particular nearly all group report negative trends for temperature.
However, for the mesosphere and lower thermosphere the reported cooling
trends fall into three different groups: The first group claims trends
to be up to 10 K/decade, the second one sees much weaker trends, of about
1-2 K/decade. The third group reports no systematic trends at all. Moreover,
positions of some authors change rather quickly. Natural variations arising
on decadal (first of all 11-year solar cycle) and longer time scales may
play a significant role in the observed/reported long-term trends due to
limited length of available data sets. With regard to trends for the mean
winds, and the seasonal and tidal harmonics, trends are reported, but their
statistical confidence is significantly lower than desired. The comparison
with models show very large differences, even with the moderate trend results.
Carbon dioxide and methane changes over the observed period do not seem
to be capable of explaining the observations with a large factor (typically
2 to 5).