Making limb and nadir measurements comparable: A common volume study of PMC brightness observed by Odin OSIRIS and AIM CIPS

Combining limb and nadir satellite observations of Polar Mesospheric Clouds (PMCs) has long been recognized as problematic due to differences in observation geometry, scattering conditions, and retrieval approaches. This study offers a method of comparing PMC brightness observations from the nadir-viewing Aeronomy of Ice in the Mesosphere (AIM) Cloud Imaging and Particle Size (CIPS) instrument and the limb-viewing Odin Optical Spectrograph and InfraRed Imaging System (OSIRIS). OSIRIS and CIPS measurements are made comparable by defining a common volume for overlapping OSIRIS and CIPS observations for two northern hemisphere (NH) PMC seasons: NH08 and NH09. We define a scattering intensity quantity that is suitable for either nadir or limb observations and for different scattering conditions. A known CIPS bias is applied, differences in instrument sensitivity are analyzed and taken into account, and effects of cloud inhomogeneity and common volume definition on the comparison are discussed. Not accounting for instrument sensitivity differences or inhomogeneities in the PMC field, the mean relative difference in cloud brightness (CIPS - OSIRIS) is −102 ± 55%. The differences are largest for coincidences with very inhomogeneous clouds that are dominated by pixels that CIPS reports as non-cloud points. Removing these coincidences, the mean relative difference in cloud brightness reduces to −6 ± 14%. The correlation coefficient between the CIPS and OSIRIS measurements of PMC brightness variations in space and time is remarkably high, at 0.94. Overall, the comparison shows excellent agreement despite different retrieval approaches and observation geometries

Measurements of thermospheric molecular oxygen from the Solar Ultraviolet Spectral Irradiance Monitor

We present a new data set of thermospheric O2 density profiles retrieved from solar occultation measurements made by the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instrument on board the UARS satellite. SUSIM is nominally a solar experiment whose mission was to measure the magnitude and variability of the UV solar irradiance. However, it was also capable of remotely sensing the Earth's upper atmosphere using occultation. SUSIM measurements of solar attenuation in the O2 Schumann Runge continuum are used to retrieve O2 density profiles between 110 and 240 kin. Between October 1991 and February 2005, SUSIM performed solar occultation measurements up to one day per week, measuring full-disk solar extinction as a function of tangent altitude at three nominal wavelengths (144, 161 and 171 nm). These data have been inverted using an optimal estimation algorithm to produce altitude profiles of O2 density. This unique data set comprises approximately 1550 measurements, which span a wide range of solar and geomagnetic activity and latitudes up to 75° in each hemisphere. We present a discussion of the SUSIM instrument, the measurement technique employed for occultations, and the operational retrieval algorithm. A full error analysis and retrieval characterization study demonstrates that the SUSIM retrievals have uncertainties of 10-15% and a vertical resolution of 5-15 km, depending on altitude and measurement wavelength. The data thus have sufficient accuracy and resolution to provide scientifically useful constraints on the O2 abundance and variability, and initial comparisons show good overall agreement between the NRLMSIS-00 model and the SUSIM measurements

Exospheric Temperature Measured by NASA-GOLD Under Low Solar Activity: Comparison With Other Data Sets

Exospheric temperature is one of the key parameters in constructing thermospheric models and has been extensively studied with in situ observations and remote sensing. The Global-scale Observations of the Limb and Disk (GOLD) at a geosynchronous vantage point provides dayglow limb images for two longitude sectors, from which we can estimate the terrestrial exospheric temperature since 2018. In this paper, we investigate climatological behavior of the exospheric temperature measured by GOLD. The temperature has positive correlations with solar and geomagnetic activity and exhibits a morning-afternoon asymmetry, both of which agree with previous studies. We have found that the arithmetic sum of F10.7 (solar) and Ap (geomagnetic) indices is highly correlated with the exospheric temperature, explaining ∼64% of the day-to-day variability. Furthermore, the exospheric temperature has good correlation with thermospheric parameters (e.g., neutral temperature, O2 density, and NO emission index) sampled at various heights above ∼130 km, in spite of the well-known thermal gradient below ∼200 km. However, thermospheric temperature at altitudes around 100 km is not well correlated with the GOLD exospheric temperature. The result implies that effects other than thermospheric heating by solar Extreme Ultraviolet and geomagnetic activity take control below a threshold altitude that exists between ∼100 and ∼130 km.Astrodynamics & Space Mission

Validation of Odin/OSIRIS stratospheric NO₂ profiles

This paper presents the validation study of stratospheric NO2 profiles retrieved from Odin/OSIRIS measurements of limb-scattered sunlight (version 2.4). The Optical Spectrograph and Infrared Imager System (OSIRIS) NO2 data set is compared to coincident solar occultation measurements by the Halogen Occultation Experiment (HALOE), Stratospheric Aerosol and Gas Experiment (SAGE) II, SAGE III, and Polar Ozone and Aerosol Measurement (POAM) III during the 2002–2004 period. Comparisons with seven Systeme d'Analyse par Observation Zenithal (SAOZ) balloon measurements are also presented. All comparisons show good agreement, with differences, both random and systematic, of less than 20% between 25 km and 35 km. Inconsistencies with SAGE III below 25 km are found to be caused primarily by diurnal effects from varying NO2 concentrations along the SAGE III line-of-sight. On the basis of the differences, the OSIRIS random uncertainty is estimated to be 16% between 15 km and 25 km, 6% between 25 km and 35 km, and 9% between 35 km and 40 km. The estimated systematic uncertainty is about 22% between 15 and 25 km, 11–21% between 25 km and 35 km, and 11–31% between 35 km and 40 km. The uncertainties for AM (sunrise) profiles are generally largest and systematic deviations are found to be larger at equatorial latitudes. The results of this validation study show that the OSIRIS NO2 profiles are well behaved, with reasonable uncertainty estimates between 15 km and 40 km. This unique NO2 data set, with more than hemispheric coverage and high vertical resolution will be of particular interest for studies of nitrogen chemistry in the middle atmosphere, which is closely linked to ozone depletion

Polar Ozone and Aerosol Measurement III measurements of water vapor in the upper troposphere and lowermost stratosphere

[1] We present water vapor measurements made by the Polar Ozone and Aerosol Measurement (POAM) III instrument since May 1998 in the upper troposphere and lowermost stratosphere. While POAM III is primarily a stratospheric instrument, many of the POAM III occultation measurements allow for the retrieval of water vapor in the upper troposphere. The Measurements of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) instruments provide a large number of coincident measurements and thus offer the best opportunity to validate POAM measurements in the highly spatially variable regions of the upper troposphere-lowermost stratosphere, where the mixing ratios are much larger than those found throughout most of the stratosphere. The comparison shows that there is no statistically significant difference in the response of the two instruments to changes in water vapor and that in the regime where the MOZAIC measurements are thought to be most accurate, the water vapor mixing ratios measured by POAM are 10% higher. The POAM III Northern Hemisphere measurements are taken from 55 to 71 and show a qualitatively reasonable seasonal variation, with high mixing ratios in the upper troposphere in the summer and low mixing ratios in the winter. Comparisons of the seasonal variations of the POAM measurements with those from the upper tropospheric Microwave Limb Sounder (MLS) measurements from the early 1990s show qualitative similarities. The similar to1 km vertical resolution of POAM measurements allows us to study in greater detail than other satellite instruments the complex variations in water vapor that occur in the upper troposphere and lowermost stratosphere. Among the interesting features observed is a rise in the level of the high-latitude hygropause from April through September