International audienceIn recent times it has become increasingly clear thatreleases of trace gases from human activity have a potentialfor causing change in the upper atmosphere. However,our knowledge of systematic changes and trends inthe temperature of the mesosphere and lower thermosphereis relatively limited compared to the Earths loweratmosphere, and not much effort has been made to synthesizethese results so far. In this article, a comprehensivereview of long-term trends in the temperature of the regionfrom 50 to 100 km is made on the basis of the availableup-to-date understanding of measurements and model calculations.An objective evaluation of the available datasets is attempted, and important uncertainly factors arediscussed. Some natural variability factors, which arelikely to play a role in modulating temperature trends,are also briefly touched upon. There are a growing numberof experimental results centered on, or consistent with,zero temperature trend in the mesopause region (80–100km). The most reliable data sets show no significant trendbut an uncertainty of at least 2 K/decade. On the otherhand, a majority of studies indicate negative trends inthe lower and middle mesosphere with an amplitude ofa few degrees (2–3 K) per decade. In tropical latitudesthe cooling trend increases in the upper mesosphere.The most recent general circulation models indicateincreased cooling closer to both poles in the middlemesosphere and a decrease in cooling toward the summerpole in the upper mesosphere. Quantitatively, thesimulated cooling trend in the middle mesosphere producedonly by CO₂ increase is usually below the observedlevel. However, including other greenhouse gasesand taking into account a “thermal shrinking” of theupper atmosphere result in a cooling of a few degreesper decade. This is close to the lower limit of the observednonzero trends. In the mesopause region, recentmodel simulations produce trends, usually below 1 K/decade,that appear to be consistent with most observationsin this regio

[1] Vertical coupling in the low-latitude atmosphere-ionosphere system driven by the 2-day wave in the tropical MLT region has been investigated. The problem is studied from an observational point of view. Three different types of data were analyzed in order to detect and extract the 2-day wave signals. The 2-day wave event during the period from 1 December 2002 to 28 February 2003 was identified in the neutral winds by radar measurements located at four tropical stations. The 2-day variations in the ionospheric electric currents (registered by perturbations in the geomagnetic field) and in the F-region electron densities were detected in the data from 23 magnetometer and seven ionosonde stations situated at low latitudes. Two features for each kind of wave were investigated in detail: the variation with time of the wave amplitude and the zonal wave number. The results show that the westward propagating global 2-day wave with zonal wave number 2 seen in the ionospheric currents and in F-region plasma is forced by the simultaneous 2-day wave activity in the MLT region. The main forcing agent in this atmosphere-ionosphere coupling seems to be the modulated tides, particularly the semidiurnal tide. This tide has a larger vertical wavelength than the diurnal tide and propagates well into the thermosphere. The parameter that appears to be affected, and thus drives the observed 2-day wave response of the ionosphere, is the dynamo electric field.

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[1] The six longest records of stratospheric aerosol (in situ measurements at Laramie, Wyoming, lidar records at: Garmisch-Partenkirchen, Germany; Hampton, Virginia; Mauna Loa, Hawaii; São José dos Campos, Brazil, and SAGE II measurements) were investigated for trend by (1) comparing measurements in the 3 volcanically quiescent periods since 1970 using standard analysis of variance techniques, and (2) analyzing residuals from a time/volcano dependent empirical model applied to entire data sets. A standard squared-error residual minimization technique was used to estimate optimum parameters for each measurement set, allowing for first order autocorrelation, which increases standard errors of trends but does not change magnitude. Analysis of variance over the 3 volcanically quiescent periods is controlled by the end points (pre-El Chichón and post-Pinatubo), and indicates either no change (Garmisch, Hampton, São José dos Campos, Laramie-0.15 mm) or a slight, statistically insignificant, decrease (Mauna Loa, Laramie-0.25 mm), À1 ± 0.5% yr À1 . The empirical model was applied to the same records plus 1020 nm SAGE II data separated into 33 latitude/altitude bins. No trend in stratospheric aerosol was apparent for 31 of 33 SAGE II data sets, 3 of 4 lidar records, and in situ measurements at 0.15 mm. For Hampton and Laramie-0.25 mm, the results suggest a weak negative trend, À2 ± 0.5% yr À1, while 2 SAGE II data sets (30-35 km, 30°and 40°N) suggest a positive trend of similar magnitude. Overall we conclude that no long-term change in background stratospheric aerosol has occurred over the period 1970-2004.

The mesospheric sodium data, obtained between 1975 and 1987 at São José dos Campos (23°S, 46°W) with a laser radar, have been analyzed in order to identify the appearance of thin sporadic sodium layers. In this search, a total of 65 events were identified. The average height of the peaks is 95.0 km. The ratio of the maximum peak density to the average layer density is normally 2.5 to 3.0, but values as high as 7 have been observed in the most outstanding cases. The events last from a few minutes to several hours, although durations of 1–2 hours are more typical. The events occur more often during periods of large meteor showers, especially in August. The diurnal variation shows an increasing number of observed peaks from 1500 LT to midnight and remains almost constant from midnight to 0600 LT. In 52 out of 54 days for which sodium and ionosonde data are available there was a sporadic E layer nearly coincident with the sodium cloud. The coincidence is good for short‐lived sporadic layers, but a substantial increase in sporadic E critical and blanketing frequencies normally precedes the long‐lasting and broader ones. These results are compatible with the suggestion that the enhanced layers are produced by the wind shear distortion of sodium clouds originating in meteor deposition, but we cannot rule out the possibility of an ion conversion mechanism.