The Earth's thermosphere and ionosphere constitute a dynamic system that varies daily in response to energy inputs from above and from below. This system can exhibit a significant response within an hour to changes in those inputs, as plasma and fluid processes compete to control its temperature, composition, and structure. Within this system, short wavelength solar radiation and charged particles from the magnetosphere deposit energy, and waves propagating from the lower atmosphere dissipate. Understanding the global-scale response of the thermosphere-ionosphere (T-I) system to these drivers is essential to advanc- ing our physical understanding of coupling between the space environment and the Earth's atmosphere. Previous missions have successfully determined how the "climate" of the T-I system responds. The Global-scale Observations of the Limb and Disk (GOLD) mission will determine how the "weather" of the T-I responds, taking the next step in understanding the coupling between the space environment and the Earth's atmosphere. Operating in geostationary orbit, the GOLD imaging spectrograph will measure the Earth's emissions from 132 to 162 nm. These measurements will be used image two critical variables-thermospheric temperature and composition, near 160 km-on the dayside disk at half-hour time scales. At night they will be used to image the evolution of the low latitude ionosphere in the same regions that were observed earlier during the day. Due to the geostationary orbit being used the mission observes the same hemisphere repeatedly, allowing the unambiguous separation of spatial and temporal variability over the Americas.

Abstract. Polar Ozone and Aerosol Measurement (POAM) III, a follow-on to the successful POAM II, is a spaceborne experiment designed to measure the vertical profiles of ozone, water vapor, nitrogen dioxide, and aerosol extinction in the polar stratosphere and upper troposphere with a vertical resolution of 1-2 km. Measurements are made by the solar occultation technique. POAM III, now in polar orbit aboard the SPOT 4 satellite, is providing data on north-and southpolar ozone phenomena, including the south-polar ozone hole, and on the spatial and temporal variability of stratospheric aerosols, polar stratospheric clouds, and polar mesospheric clouds.

[1] Within the SPARC Data Initiative, the first comprehensive assessment of the quality of 13 water vapor products from 11 limb-viewing satellite instruments (LIMS, SAGE II, UARS-MLS, HALOE, POAM III, SMR, SAGE III, MIPAS, SCIAMACHY, ACE-FTS, and Aura-MLS) obtained within the time period 1978-2010 has been performed. Each instrument's water vapor profile measurements were compiled into monthly zonal mean time series on a common latitude-pressure grid. These time series serve as basis for the "climatological" validation approach used within the project. The evaluations include comparisons of monthly or annual zonal mean cross sections and seasonal cycles in the tropical and extratropical upper troposphere and lower stratosphere averaged over one or more years, comparisons of interannual variability, and a study of the time evolution of physical features in water vapor such as the tropical tape recorder and polar vortex dehydration. Our knowledge of the atmospheric mean state in water vapor is best in the lower and middle stratosphere of the tropics and midlatitudes, with a relative uncertainty of˙2-6% (as quantified by the standard deviation of the instruments' multiannual means). The uncertainty increases toward the polar regions (˙10-15%), the mesosphere (˙15%), and the upper troposphere/lower stratosphere below 100 hPa (˙30-50%), where sampling issues add uncertainty due to large gradients and high natural variability in water vapor. The minimum found in multiannual (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) mean water vapor in the tropical lower stratosphere is 3.5 ppmv (˙14%), with slightly larger uncertainties for monthly mean values. The frequently used HALOE water vapor data set shows consistently lower values than most other data sets throughout the atmosphere, with increasing deviations from the multi-instrument mean below 100 hPa in both the tropics and extratropics. The knowledge gained from these comparisons and regarding the quality of the individual data sets in different regions of the atmosphere will help to improve model-measurement comparisons (e.g., for diagnostics such as the tropical tape recorder or seasonal cycles), data merging activities, and studies of climate variability.

The NASA Global‐scale Observations of the Limb and Disk (GOLD) mission has flown an ultraviolet‐imaging spectrograph on SES‐14, a communications satellite in geostationary orbit at 47.5°W longitude. That instrument observes the Earth's far ultraviolet (FUV) airglow at ~134–162 nm using two identical channels. The observations performed include limb scans, stellar occultations, and images of the sunlit and nightside disk from 6:10 to 00:40 universal time each day. Initial analyses reveal interesting and unexpected results as well as the potential for further studies of the Earth's thermosphere‐ionosphere system and its responses to solar‐geomagnetic forcing and atmospheric dynamics. Thermospheric composition ratios for major constituents, O and N2, temperatures near 160 km, and exospheric temperatures are retrieved from the daytime observations. Molecular oxygen (O2) densities are measured using stellar occultations. At night, emission from radiative recombination in the ionospheric F region is used to quantify ionospheric density variations in the equatorial ionization anomaly (EIA). Regions of depleted F region electron density are frequently evident, even during the current solar minimum. These depletions are caused by the “plasma fountain effect” and are associated with the instabilities, scintillations, or “spread F” seen in other types of observations, and GOLD makes unique observations for their study.

[1] This paper describes the operational version 3 algorithms used to analyze data from the Polar Ozone and Aerosol Measurement (POAM) III instrument. We include a thorough discussion of both the forward model and retrieval algorithms, including the algorithms used to derive altitude information and normalize the measured radiances to produce atmospheric transmission profiles. The operational POAM III algorithms retrieve O 3 , NO 2 , H 2 O, and O 2 (or total) density, as well as aerosol extinction between 353 and 1018 nm. All atmospheric species are retrieved simultaneously using the technique of optimal estimation. The conversion of transmission data to geophysical profiles is achieved via a two-step process, beginning with a spectral inversion to partition the various gas and aerosol components of the measured slant optical depth, followed by a spatial inversion to produce altitude profiles of gas density and aerosol extinction from the path-integrated quantities. A formal error analysis is also presented, yielding estimates of the total random error budget for the retrieved profiles. In addition to the theoretical error analysis, we present results from empirical analyses that quantify error components due to aerosol feedback in the gas retrievals, as well as sunspot artifacts that are sometimes present in the data. Finally, we present a quantitative retrieval characterization based on analysis of the full retrieval averaging kernel matrix. This matrix is used to calculate vertical resolution profiles for all retrieved species, as well as to quantify the information content of the retrievals, including the a priori weighting in the retrievals, the coupling between the various retrieval species, and the spectral resolution of the aerosol retrievals.

[1] This paper describes the validation of ozone profiles from the Polar Ozone and Aerosol Measurement (POAM) III instrument. POAM III O 3 is measured with 1-km vertical resolution throughout most of the stratosphere and random errors of $5%. It is shown that sunspots do not significantly affect the POAM O 3 retrievals, nor do polar stratospheric clouds, except under rarely encountered, extreme conditions of very low O 3 and exceptionally high aerosol extinction. A statistical analysis is presented of comparisons between coincident measurements from POAM III and ozonesondes, the Halogen Occultation Experiment (HALOE) and the Stratospheric Aerosol and Gas Experiment (SAGE) II. On average, POAM III O 3 profiles agree to within ±5% with these correlative data from 13 to 60 km. There is a suggestion that from 30 to 60 km POAM III sunrise data might be biased slightly low (<5%) relative to POAM sunset data. There is evidence that POAM III has a high bias of up to $0.1 ppmv from 10 to 12 km, and that this bias might stem, in part, from errors in the retrieval of aerosol extinction at 0.6 mm, the primary O 3 absorption wavelength in the POAM retrievals. Below 10 km the POAM III data agree with coincident sonde measurements to better than 0.05 ppmv on average, which can correspond to large relative differences of more than +30% at 8-9 km and À100% at 5 km. We conclude that the POAM III profiles are highly accurate and adequate for quantitative scientific studies.

[1] A comprehensive quality assessment of the ozone products from 18 limb-viewing satellite instruments is provided by means of a detailed intercomparison. The ozone climatologies in form of monthly zonal mean time series covering the upper troposphere to lower mesosphere are obtained from LIMS, SAGE I/II/III, UARS-MLS, HALOE, POAM II/III, SMR, OSIRIS, MIPAS, GOMOS, SCIAMACHY, ACE-FTS, ACE-MAESTRO, Aura-MLS, HIRDLS, and SMILES within 1978-2010. The intercomparisons focus on mean biases of annual zonal mean fields, interannual variability, and seasonal cycles. Additionally, the physical consistency of the data is tested through diagnostics of the quasi-biennial oscillation and Antarctic ozone hole. The comprehensive evaluations reveal that the uncertainty in our knowledge of the atmospheric ozone mean state is smallest in the tropical and midlatitude middle stratosphere with a 1σ multi-instrument spread of less than ±5%. While the overall agreement among the climatological data sets is very good for large parts of the stratosphere, individual discrepancies have been identified, including unrealistic month-to-month fluctuations, large biases in particular atmospheric regions, or inconsistencies in the seasonal cycle. Notable differences between the data sets exist in the tropical lower stratosphere (with a spread of ±30%) and at high latitudes (±15%). In particular, large relative differences are identified in the Antarctic during the time of the ozone hole, with a spread between the monthly zonal mean fields of ±50%. The evaluations provide guidance on what data sets are the most reliable for applications such as studies of ozone variability, model-measurement comparisons, detection of long-term trends, and data-merging activities.

Abstract. We describe, for the first time, measurements of stratospheric nitrogen dioxide (NO2) by the Polar Ozone and Aerosol Measurement (POAM II) instrument. Measurements span October 1993 through mid-November 1996 and cover latitude ranges from 55 ø to 72øN and from 63 ø to 88øS. Comparisons with coincident satellite and space shuttle observations show good agreement and confirm the validity of POAM II measurements for scientific investigations. Overall seasonal variations in both hemispheres are qualitatively consistent with standard photochemistry. In the austral late winter/early spring of 1994, however, anomalously high NO2 mixing ratios were observed above 22 km. We conclude that these high NO2 levels resulted from downward transport of NOx-enhanced air from the mesosphere or thermosphere inside the polar vortex. Enhanced NO2 mixing ratios in 1994 were factors of-l.3 and 2.5 larger than at the corresponding times in 1995 and 1996. We conclude that POAM II observations of coincident, localized reductions in ozone of up to 40% were caused by the increased stratospheric NOx via the standard catalytic NO• cycle.