Marianna G. Shepherd

Review of mesospheric temperature trends

In recent times it has become increasingly clear that releases of trace gases from human activity have a potential for causing change in the upper atmosphere. However, our knowledge of systematic changes and trends in the temperature of the mesosphere and lower thermosphere is relatively limited compared to the Earths lower atmosphere, and not much effort has been made to synthesize these results so far. In this article, a comprehensive review of long-term trends in the temperature of the region from 50 to 100 km is made on the basis of the available up-to-date understanding of measurements and model calculations. An objective evaluation of the available data sets is attempted, and important uncertainly factors are discussed. Some natural variability factors, which are likely to play a role in modulating temperature trends, are also briefly touched upon. There are a growing number of experimental results centered on, or consistent with, zero temperature trend in the mesopause region (80–100 km). The most reliable data sets show no significant trend but an uncertainty of at least 2 K/decade. On the other hand, a majority of studies indicate negative trends in the lower and middle mesosphere with an amplitude of a few degrees (2–3 K) per decade. In tropical latitudes the cooling trend increases in the upper mesosphere. The most recent general circulation models indicate increased cooling closer to both poles in the middle mesosphere and a decrease in cooling toward the summer pole in the upper mesosphere. Quantitatively, the simulated cooling trend in the middle mesosphere produced only by CO 2 increase is usually below the observed level. However, including other greenhouse gases and taking into account a “thermal shrinking” of the upper atmosphere result in a cooling of a few degrees per decade. This is close to the lower limit of the observed nonzero trends. In the mesopause region, recent model simulations produce trends, usually below 1 K/decade, that appear to be consistent with most observations in this region

The Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite: A 20 year perspective

The Wind Imaging Interferometer (WINDII) was launched on the NASAs Upper Atmosphere Research Satellite on 12 September 1991 and operated until 2003. Its role in the mission was to measure vector winds in the Earths atmosphere from 80 to 110 km, but its measurements extended to nearly 300 km. The approach employed was to measure Doppler shifts from a suite of visible region airglow lines emitted over this altitude range. These included atomic oxygen O(1S) and O(1D) lines, as well as lines in the OH Meinel (8,3) and O2 Atmospheric (0,0) bands. The instrument employed was a Doppler Michelson Interferometer (DMI) that measured the Doppler shift as a phase shift of the cosinusoidal interferogram generated by single airglow lines. An extensive validation program was conducted after launch to confirm the accuracy of the measurements. The dominant wind field, the first one observed by WINDII, was that of the migrating diurnal tide at the equator. The overall most notable WINDII contribution followed from this; determining the influence of dynamics on the transport of atmospheric species. Currently, non-migrating tides are being studied in the thermosphere at both equatorial and high latitudes. Other aspects investigated included solar and geomagnetic influences, temperatures from atmospheric scale heights, nitric oxide concentrations and the occurrence of polar mesospheric clouds. The results of these observations are reviewed from a perspective of twenty years. A future perspective is then projected, involving more recently developed concepts. It is intended that this description will be helpful for those planning future missions.

Global variability of mesospheric temperature : Mean temperature field

[1] Daytime zonally (longitudinally) averaged temperatures from the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS) and nightly temperatures from various ground-based hydroxyl airglow observations are employed in the study of the global and seasonal variability of the upper mesospheric temperature field. The study examines the latitudinal variability of the annual cycle of mesospheric temperature at 75, 82, and 87 km employing 7 years (1991-1997) of WINDII mesospheric temperature data at latitudes from 20°S to 65°N at 75 km, 35°S to 65°N at 82 km, and from 45°S to 65°N at 87 km height. Particular attention is given to the latitude region of ±40° around the equator. Harmonic analysis of the 7-year temperature time series reveals the presence of a dominant annual, ∼90- and 60-day oscillations at high northern latitudes and a strong semiannual oscillation (SAO) at equatorial and tropical latitudes. A quasi-biennial oscillation (QBO) is also identified extending from 45°S to 65°N. At 75 km the SAO is manifested as minima in the temperature composites at spring and fall equinox and maxima at winter and summer solstice; at 87 km the SAO is out of phase with respect to the 75-km SAO, with maxima at equinox and minima around the solstice periods. The phase reversal takes place around 82 km and is associated with a mesospheric temperature inversion between 77 and 86 km height. Accounting for tidal contribution by adopting tidal predictions by the Extended Canadian Middle Atmosphere Model (CMAM) shows that a strong temperature decrease (∼35 K) seen during the 1993 March equinox at equatorial and tropical latitudes is not associated with solar migrating tides. WINDII global climatology derived at 75, 82, and 87 km revealed mesospheric SAO asymmetry with a stronger September equinox and interhemispheric asymmetry with a quieter and colder southern hemisphere. Comparisons with independent ground-based observations and the Solar Mesospheric Explorer (SME) satellite data are also presented showing good to excellent agreement in the derived annual and SAO parameters. The results presented provide the first high-vertical-and-temporal resolution global daytime temperature climatology in the upper mesosphere and in the vicinity of the mesopause.

Springtime transition in upper mesospheric temperature in the northern hemisphere

Abstract Airglow emission observations by the wind imaging interferometer (WINDII) on the Upper Atmosphere Research Satellite and three optical ground-based stations previously revealed a “springtime transition” in atomic oxygen. The transition is characterized by a rapid 2-day rise in the night-time oxygen nightglow emission rate by a factor of 2 to 3 followed by a subsequent decrease by a factor of 10 in the same period of time indicating a depletion of atomic oxygen that persists for days. The current study examines signatures in the upper mesosphere temperature field (70– 95 km height range), derived from the WINDII Rayleigh scattering observations, which may be associated with this springtime depletion of the atomic oxygen. Comparisons with ground-based OH airglow rotational temperatures, Na lidar and Rayleigh scattering lidar temperatures, and meteor radar temperatures at middle and high latitudes in the Northern Hemisphere are presented and discussed. Data from the northern springtimes in 1992 and 1993 are reported upon in detail. It was found that all datasets used in the study agree well with each other taking account of the day/night time mean differences. A rapid temperature enhancement was observed at spring equinox at northern midlatitudes followed by a period of mean temperature colder than the one observed prior to the enhancement event, a pattern similar to that associated with the “springtime transition” observed in the oxygen emissions. The enhancement was also revealed in the average annual temperatures at 87 km , obtained by combining observations from 1992 to 1996, and in more recent temperature data from 1998 and 1999 at mid- and high northern latitudes. The results suggest that the temperature enhancement is associated with the last stratospheric warming event, observed at the end of March and early April.

Upper mesosphere temperatures in summer: WINDII observations and comparisons

Atmospheric temperature measurements from the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS), determined from the observed Rayleigh scattering are presented. Infrared radiances predicted from these profiles are compared with measurements from the Improved Stratospheric and Mesospheric Sounder on UARS for nine cases of spatial and temporal coincidence with an effective difference of about 5–10 K in the height range 70–85 km. A comparison with monthly averaged SME results and Rayleigh lidar temperature observations at mid-latitudes taken 8 years earlier indicated that WINDII temperatures were cooler on average by 10 K; this is consistent with a temperature decrease of 1.5 K/year inferred from the SME and lidar results. At high latitudes the agreement between falling spheres and WINDII was found to be very good.

Planetary scale and tidal perturbations in mesospheric temperature observed by WINDII

WINDII, the Wind Imaging Interferometer on the Upper Atmospheric Research Satellite measures winds and emission rates from selected excited metastable species. Here we report on measurements of the atmospheric Rayleigh scattering from the O(1S) background filter at 553 nm wavelength used to derive temperature profiles in the upper mesosphere from 70 km to 95 km, for solstice periods from December 1992/93 and January 1993/94. The data are first zonally averaged and then combined in local time over about one month. Based on these temperatures, an analysis of planetary wave structures and tidal perturbations employing least-mean-square (LMS) fits to the data has been conducted and the results are presented. The planetary wave structures observed were well described with a quasi two-day wave (QTDW). Amplitudes of 14 K and 10 K at 85 km height for downleg (descending) and upleg (ascending) sampling respectively at latitudes from 20°S to 40°S were found to be in good agreement with QTDW temperature results from the MLS/UARS experiment assuming a vertical amplitude structure of the type described by the HRDI/UARS mesospheric wind observations. It is shown that the diurnal tide amplitudes estimated from latitudes from 25°N to 35°S using the LMS fit maximize at the equator with an amplitude of about 6 K and decrease toward tropical latitudes, consistent with the classical tidal theory and predictions from the TIME-GCM model.

Evidence of non-LTE in the CO2 15 µm weak bands from ISAMS and WINDII observations

An analysis of the measurements of the CO2 15 µm radiance emission by the UARS/ISAMS 30W channel and the kinetic temperature from UARS/WINDII taken on July 21, 1992 at northern latitudes around the mesopause is presented. The modeling of the measurements clearly show evidence of non-LTE emissions in the CO2(ν2=1) levels of the minor isotopes 636, 628, and 627 in the 70–90 km region. A comparison with non-LTE model predictions by Lopez-Puertas et al. [1992a] shows a good agreement within the errors in the measured quantities. This constitutes the first experimental evidence of non-LTE emissions in these CO2 15 µm weak bands. The measurements also represent indirect evidence of the net radiative heating produced by these bands around the summer mesopause.

WINDII observations of thermospheric O(1D) nightglow emission rates, temperature and wind: 1. The Northern hemisphere midnight temperature maximum and the wave 4†

The Midnight Temperature Maximum (MTM) is a large scale neutral temperature anomaly with a wide-ranging effect on the nighttime thermospheric dynamics at low latitudes. The focus of the current study is an investigation of the extent of the MTM to northern midlatitudes (20°N – 40°N) employing multi-year observations of O(1D) airglow volume emission rates (VER), Doppler temperatures (DoT), and neutral winds over the altitude range of 190–300 km by the Wind Imaging Interferometer (WINDII) experiment on board the Upper Atmosphere Research Satellite. The MTM dependence on longitude, season, local time and altitude was examined. Midnight maxima were observed both in the O(1D) VER and DoT with peaks at ~24 LT during winter solstice, 22 LT for fall equinox and 2 LT for spring equinox. The peak in the DoTs was marked with strong southward meridional winds (e.g. ~ 100 − 150 m s-1). Latitude/longitude maps of the VER and DoT revealed wave-4 signatures most persistently seen around local midnight, with very little variation in phase, while the amplitude of the individual peaks varied with time. The observed perturbations in the O(1D) VER and temperature were out-of-phase with respect to longitude. Two of the peaks at ~ 100°E and 260°E – 300°E were almost stationary, while the other two peaks varied in strength over the period of observation. A common feature was that one of the wave 4 peaks was always over the American sector, it was constant with local time and the meridional wind was southward only over the same region.

Advancing the understanding of the Sun- Earth interaction—the Climate and Weather of the Sun-Earth System (CAWSES) II program

The Scientific Committee on Solar–Terrestrial Physics (SCOSTEP) of the International Council for Science (ICSU) implemented an international collaborative program called Climate and Weather of the Sun–Earth System (CAWSES), which was active from 2004 to 2008; this was followed by the CAWSES II program during the period of 2009–2013. The CAWSES program was aimed at improving the understanding of the coupled solar–terrestrial system, with special emphasis placed on the short-term (weather) and long-term (climate) variability of solar activities and their effects on and responses of Geospace and Earth’s environment. Following the successful implementation of CAWSES, the CAWSES II program pursued four fundamental questions addressing the way in which the coupled Sun–Earth system operates over time scales ranging from minutes to millennia, namely, (1) What are the solar influences on the Earth’s climate? (2) How will Geospace respond to an altered climate? (3) How does short-term solar variability affect the Geospace environment? and (4) What is the Geospace response to variable inputs from the lower atmosphere? In addition to these four major tasks, the SCOSTEP and CAWSES promoted E-science and informatics activities including the creation of scientific databases and their effective utilization in solar–terrestrial physics research. Capacity building activities were also enhanced during CAWSES II, and this represented an important contribution of SCOSTEP to the world’s solar–terrestrial physics community. This introductory paper provides an overview of CAWSES II activities and serves as a preface to the dedicated review papers summarizing the achievements of the program’s four task groups (TGs) and the E-science component.