Upper Atmosphere Research Satellite Research Articles

Retrieval of stratospheric ozone profiles from MIPAS/ENVISAT limb emission spectra: a sensitivity study

Abstract. We report on the dependence of ozone volume mixing ratio profiles, retrieved from limb emission infrared spectra of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), on different retrieval setups such as the treatment of the background continuum, cloud filtering, spectral regions used for analysis and a series of further more technical parameter choices. The purpose of this investigation is to better understand the error sources of the ozone retrieval, to optimize the current retrieval setup and to document changes in the data versions. It was shown that the cloud clearing technique used so far (cloud index 1.8) does not reliably exclude all cloud-contaminated spectra from analysis. Through analysis of spectra calculated for cloudy atmospheres we found that the cloud index should be increased to a value of 3.0 or higher. Further, it was found that assignment of a common background continuum to adjacent microwindows within 5 cm−1 is advantageous, because it sufficiently represents the continuum emission by aerosols, clouds and gases as reported in the literature, and is computationally more efficient. For ozone retrieval we use ozone lines from MIPAS band A (685–970 cm−1) and band AB (1020–1170 cm−1) as well. Therefore we checked ozone retrievals with lines from bands A or AB only for a systematic difference. Such a difference was indeed found and could, to a major part, be attributed to the spectroscopic data used in these two bands, and to a minor part to neglection of modelling of non-local thermodynamic (non-LTE) emissions. Another potential explanation, a bias in the radiance calibration of level-1B spectra of bands A and AB, could largely be ruled out by correlation analysis and inspection of broadband spectra. Further upgrades in the ozone retrieval consist of application of an all-zero a-priori profile and a weaker regularization. Finally, the ozone distribution obtained with the new retrieval setup (data versions V3o_O3_7) was compared to the data version used before (V2_O3_2). Differences are smaller than ±0.4 ppmv in the altitude region 15–50 km. Further, differences to ozone measured by the HALogen Occultation Experiment (HALOE) on the Upper Atmospheric Research Satellite (UARS) are partly reduced with the new MIPAS data version.

Solar Tides as Revealed by Measurements of Mesosphere Temperature by the MLS Experiment on UARS

Abstract Temperatures between 25 and 86 km measured by the Microwave Limb Sounder (MLS) experiment on the Upper Atmosphere Research Satellite (UARS) are analyzed to delineate diurnal, semidiurnal, and terdiurnal tidal structures and stationary planetary waves. These Fourier components are determined from temperatures averaged in bins covering 5° latitude, 30° longitude and 1 h in local time. This study confirms the presence of diurnal nonmigrating tides with zonal wavenumbers s = 0, 2, −3 [s > 0 (s < 0) implying westward (eastward) propagation] and semidiurnal tides with s = 1 and 3, and some components of lesser importance that were previously determined from UARS wind measurements near 95 km. The seasonal–latitudinal and height structures of these components are now revealed, and utilized to aid in interpreting their behaviors and ascertaining their origins. New discoveries include the terdiurnal s = 2 and s = 4 components, and trapped nonmigrating diurnal tides with s = 0 and s = 2. The former are likely to arise from nonlinear interaction between the migrating (s = 3) terdiurnal tide and the stationary planetary wave with s = 1. The latter may reflect the presence of a longitude-dependent in situ heat source, or in situ nonlinear interaction between the migrating diurnal tide and a stationary planetary wave with s = 1. The present results provide a rich mixture of observational results to challenge both mechanistic and general circulation models of the middle atmosphere. In addition, internal consistency is established between the MLS tidal temperatures at 86 km and previously derived tidal winds at 95 km within the context of tidal theory. This result represents one step in the validation of measurements required for successful application of data-model assimilation techniques to the mesosphere and lower thermosphere.

Perturbed profiles of oxygen nightglow emissions as observed by WINDII on UARS

Perturbed profiles having multiple peaks in the O( 1S) green line, O 2(b 1∑ g +) Atmospheric (0–0) band, and OH (8–3) Meinel band P 1(3) line nightglow emissions in the mesopause region are studied using observations of the Wind Imaging Interferometer (WINDII) onboard the Upper Atmosphere Research Satellite (UARS). These profiles were frequently observed by WINDII over the study period of 1991–1997, comprising up to 40% of the observations in bins having certain latitudes and local times. The preferred locations and times for their occurrences coincide with steep emission rate gradients associated with the diurnal tide. The assumption of horizontal homogeneity in the WINDII data processing implies that the perturbed profiles may be caused by horizontal as well as vertical variations. A 1-D model is developed to simulate the horizontal variation of volume emission rate embedded in a Gaussian distributed emission profile. The model results show that the secondary peak in the vertical emission profile could be the reflection of a wave-like horizontal structure with wavelengths between 100 and 400 km. The features of the horizontal structures correspond to localized regions of gravity waves with extents up to 2000 km and lifetimes of one or more days. It is concluded that the WINDII perturbed emission profiles could be produced by horizontal variations induced by gravity waves with wavelengths between 100 and 400 km, and that these occur in preferred regions of steep emission rate gradients associated with the diurnal tide.

Performance evaluation of a thermal Doppler Michelson interferometer system

The thermal Doppler Michelson interferometer is the primary element of a proposed limb-viewing satellite instrument called SWIFT (Stratospheric Wind Interferometer for Transport studies). SWIFT is intended to measure stratospheric wind velocities in the altitude range of 15-45 km. SWIFT also uses narrowband tandem etalon filters made of germanium to select a line out of the thermal spectrum. The instrument uses the same technique of phase-stepping interferometry employed by the Wind Imaging Interferometer onboard the Upper Atmosphere Research Satellite. A thermal emission line of ozone near 9 microm is used to detect the Doppler shift due to winds. A test bed was set up for this instrument that included the Michelson interferometer and the etalon filters. For the test bed work, we investigate the behavior of individual components and their combination and report the results.

Inter-annual variation of gravity waves in the Arctic and Antarctic winter middle atmosphere

High latitude, middle atmospheric (28–80 km) gravity wave observations from the Microwave Limb sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) are available during five Arctic and Antarctic winters between 1991 and 1997. Newer data from NOAA Advanced Microwave Sounding Unit (AMSU) instruments have been introduced recently, in order to extend the gravity wave measurements from 1998 to present. Analyses of MLS and AMSU data show substantial inter-annual variability in both magnitude and spatial patterns of gravity wave activity at both Arctic and Antarctic latitudes.

A space-based climatology of diurnal MLT tidal winds, temperatures and densities from UARS wind measurements

A self-consistent global tidal climatology, useful for comparing and interpreting radar observations from different locations around the globe, is created from space-based Upper Atmosphere Research Satellite (UARS) horizontal wind measurements. The climatology created includes tidal structures for horizontal winds, temperature and relative density, and is constructed by fitting local (in latitude and height) UARS wind data at 95 km to a set of basis functions called Hough mode extensions (HMEs). These basis functions are numerically computed modifications to Hough modes and are globally self-consistent in wind, temperature, and density. We first demonstrate this self-consistency with a proxy data set from the Kyushu University General Circulation Model, and then use a linear weighted superposition of the HMEs obtained from monthly fits to the UARS data to extrapolate the global, multi-variable tidal structure. A brief explanation of the HMEs’ origin is provided as well as information about a public website that has been set up to make the full extrapolated data sets available.

Technical note: A stratospheric climatology for O<sub>3</sub>, H<sub>2</sub>O, CH<sub>4</sub>, NO<sub>x</sub>, HCl and HF derived from HALOE measurements

Abstract. The Halogen Occultation Experiment (HALOE) on board the Upper Atmosphere Research Satellite (UARS) has observed mixing ratios of important trace species in the stratosphere for more than a decade since 1991. Here we present a climatology for the stratosphere compiled from HALOE O3, H2O, CH4, NOx, HCl, and HF data for the period from 1991 to 2002. In this approach, the data are averaged over equivalent latitude instead of latitude in order to correctly reproduce the gradients at the transport barriers like the polar vortex edge. The climatology is compiled for 5 degree equivalent latitude bins. Seasonal dependence is taken into account by choosing intervals of one month. The climatology is available as an electronic supplement.

Solar–Stellar Irradiance Comparison Experiment II (Solstice II): Instrument Concept and Design

The Solar-Stellar Irradiance Comparison Experiment II (SOLSTICE II) is one of four experiments launched aboard the Solar Radiation and Climate Experiment (SORCE) on 25 January, 2003. Its principal science objectives are to measure solar spectral irradiance from 115 to 320 nm with a spectral resolution of 1 nm, a cadence of 6 h, and an accuracy of 5% and to determine solar variability with a relative accuracy of 0.5% per year during a 5-year long nominal mission. SOLSTICE II meets these objectives using a pair of identical scanning grating monochromators that can measure both solar and stellar irradiance. Instrument radiometric responsivity was calibrated to ∼3% absolute accuracy before launch using the Synchrotron Ultraviolet Radiation Facility (SURF) at the National Institute for Standards and Technology (NIST) in Gaithersburg, MD. During orbital operations, SOLSTICE II has been making daily measurements of both the Sun and an ensemble of bright, stable, main-sequence B and A stars. The stellar measurements allow the tracking of changes in instrument responsivity with a relative accuracy of 0.5% per year over the life of the mission. SOLSTICE II is an evolution of the SOLSTICE I instrument that is currently operating on the Upper Atmosphere Research Satellite (UARS). This paper reviews the basic SOLSTICE concept and describes the design, operating modes, and early performance of the SOLSTICE II instrument.

Validation of stratospheric temperatures measured by Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard the Envisat satellite provides temperature and various gas profiles from limb‐viewing midinfrared emission measurements. The stratospheric temperatures retrieved at the Institut für Meteorologie und Klimaforschung (IMK) for September/October 2002 and October/November 2003 are compared with a number of reference data sets, including global radiosonde (RS) observations, radio occultation (RO) measurements of Global Positioning System (GPS) on German Challenging Minisatellite Payload (CHAMP) and Argentinean Satelite de Aplicaciones Cientificas‐C (SAC‐C) satellite, Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS), and the analyses of European Centre for Medium‐Range Weather Forecasts (ECMWF) and Met Office (METO), United Kingdom. The data sets show a good general agreement. Between 10 and 30 km altitude the mean differences are within ±0.5 K for the averages over the height interval and within ±(1–1.5) K at individual levels for comparisons with RS, GPS‐RO/CHAMP, and SAC‐C, ECMWF, and METO. Between 30 and 45 km the MIPAS mean temperatures, averaged over the height region, are higher than ECMWF but lower than METO by ∼1.5 K, while they differ by ±0.5 K with respect to HALOE, with maximum discrepancies of ∼2.5 K peaking around 35 km. Between 45 and 50 km, MIPAS temperatures show a low bias compared to HALOE, ECMWF, and METO with mean differences of −1 to −3 K and with a better agreement with HALOE. The large discrepancies between MIPAS and the analyses above 30 km likely suggest deficiency in the underlying general circulation models. The standard deviations vary between 2.5 and 3.5 K for individual data sets, with more than 70% being contributed from the expected variability of the atmosphere. Retrieved temperatures with accuracy of ∼0.5–1 K after removing the atmospheric variability provide highly accurate knowledge to characterize our environment.

Longitudinal variability of water vapor and cirrus in the tropical tropopause layer

The Microwave Limb Sounder (MLS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) are both instruments on the Upper Atmosphere Research Satellite and are sensitive to water vapor and subvisible cirrus in the tropical tropopause layer. We analyze the longitudinal and vertical variations of water vapor in the tropical tropopause layer using a new version of MLS water vapor that has improved vertical resolution. The water vapor fields show similarity with subvisible cirrus and sea surface temperature (SST) on seasonal and intraseasonal timescales. The formation of cirrus is more prevalent over continental landmasses and over warm oceanic areas where the upper troposphere is also moist. The intraseasonal variability in cirrus is correlated with upper tropospheric water vapor and linked to a variability in SST of about 1°C. The incidence of cirrus increases with SST until temperatures exceed about 29°C. A further rise in sea surface temperature does not see a corresponding increase in cirrus. Similarly, the incidence of cirrus increases as 100 hPa temperatures fall, with clouds being more likely when temperatures are about 190K. At temperatures colder than this, when in situ formation of cirrus should be more likely, there is a decrease in the likelihood of finding cirrus. Thus evidence points strongly to a dominant role for the convective formation of these cirrus.

On the response of the O(1S) dayglow emission rate to the Sun's energy input: An empirical model deduced from WINDII/UARS global measurements

More than 520,000 emission rate profiles of the O(1S) dayglow (557.7 nm, the atomic oxygen green line) were measured by the Wind Imaging Interferometer (WINDII) on the Upper Atmospheric Research Satellite (UARS) during 1991–1997, providing an unprecedented and unique resource for studying the O(1S) emission layer, its related physics and chemistry, and the response of the mesosphere and thermosphere to the solar input. The daytime O(1S) emission is one of the most remarkable and persistent phenomena in the Earth's atmosphere between 80 and 280 km. The emission has two components, peaking at 140–180 km and 94–104 km. WINDII measurements show that both components are sensitively correlated with the Sun in the sense that (1) for a given day, the peak emission rates in both the F‐region and the E‐region increase with increasing cosine of the solar zenith angle, the height of the peak emission rate in the F‐region decreases with increasing peak emission rate but that in the E‐region does not have a clear relationship with the peak emission rate; (2) both the peak emission rate and its height in the F‐region as well in the E‐region increase with increasing solar irradiance, i.e., they follow the solar cycle. For the first time an empirical model of the green line emission rate and height of the peak emission are derived from WINDII global measurements over six years as functions of the solar zenith angle and the solar irradiance using the daily solar F10.7 cm flux as a proxy. This model provides a baseline for the unperturbed daytime green line emission for any time and any location. Owing to the direct effect of the solar zenith angle, the emission rate is symmetrical with respect to local noon; and globally, it is symmetrical with respect to the equator at equinox, and is much larger in the summer hemisphere than in the winter hemisphere at solstice. As a result, for a constant solar irradiance, the emission rate has an annual oscillation at mid and high latitudes, and a semiannual oscillation at the equator. In the solar cycle 22, for an overhead Sun, the max/min ratio of the F10.7 cm flux is about four, and the max/min ratio of the integrated O(1S) emission rate is about three. Some unsolved problems regarding the mechanism of the green line emission are reported.

Large-scale circulation of atomic oxygen in the upper mesosphere and lower thermosphere

The variability of the atomic oxygen green line airglow at 557.7 nm, originating from the O( 1S) level, has a long history of observation. Only recently, global observations of the oxygen airglow, interpreted with the help of global circulation models have provided some understanding. Satellite observations of winds and temperatures clearly demonstrate the dynamical influence of tides on the daily variations. Both annual and semi-annual components of the variation have been identified as occurring over the course of the year. From the large-scale circulation of the atmosphere one expects downwelling in the winter at high latitudes, causing enhanced atomic oxygen and enhanced airglow while the opposite is expected in summer. In the present investigation a search is made for this large-scale signature using data from the WIND Imaging Interferometer on the Upper Atmosphere Research Satellite, model results from the Thermosphere–Ionosphere–Mesosphere-Electrodynamics-Global Circulation Model, and earlier published results. A well-defined semi-annual variation of emission rate is found in the tropics, apparently the result of the semi-annual variation of the diurnal tide. Annual and semi-annual patterns are found at mid-latitudes, in satellite and model data, while ground-based observations detect only an annual variation, with a maximum in the autumn. At still higher latitudes the fall peak persists, but with a deep depletion of atomic oxygen in the springtime; this appears to be the signature of the large-scale circulation.

Zonal wind velocity profiles in the equatorial electrojet derived from phase velocities of type II radar echoes

[1] Zonal wind profiles in the daytime equatorial electrojet are inferred from the Doppler shifts of type II radar echoes observed at the Jicamarca Radio Observatory (JRO) in Peru. The inference is based on a three-dimensional electrostatic potential model. The model includes anomalous effects and is constrained by radar and magnetometer data. The amplitude and phase of the calculated zonal wind profiles are in general agreement with representative wind profiles measured by the WINDII instrument on board the Upper Atmosphere Research Satellite (UARS). The calculated winds also have the same general characteristics as zonal wind profiles measured by rocket-borne chemical release experiments. However, the magnitude of the latter are larger than the former. The temporal behavior of the calculated zonal winds suggests a downward phase progression with a roughly semidiurnal period.

Interannual and seasonal variations of diurnal tide, gravity wave, ozone, and water vapor as observed by MLS during 1991–1994

The diurnal tide in the mesosphere and lower thermosphere (MLT) shows large seasonal and interannual variations. Despite recent modeling investigations, the underlying physical mechanisms for causing these variations remain unclear. This paper provides further observational constraints to tide-sensitive variables (H 2O, O 3, and gravity wave variances) used by the models, which are obtained simultaneously by upper atmosphere research satellite microwave limb sounder at altitudes below the MLT region. The strong quasi biannual oscillation and semiannual oscillation variations in these measurements reveal good correlations between the diurnal tide with other tide-sensitive variables, which should be taken into account for further modeling studies.

Global variability of mesospheric temperature: Mean temperature field

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.

Elevated ozone in the troposphere over the Atlantic and Pacific oceans in the Northern Hemisphere

Tropospheric column ozone (TCO) is derived from differential measurements of total column ozone from Total Ozone Mapping Spectrometer (TOMS), and stratospheric column ozone (SCO) from the Microwave Limb Sounder (MLS) instrument on the Upper Atmosphere Research Satellite (UARS). It is shown that TCO during late spring and summer months over the Atlantic and Pacific oceans at northern mid‐latitudes is about 50–60 Dobson Units (DU) which is about the same as over the continents of North America, Europe and Asia (except high altitude mountain regions), where surface emissions of NOx from industrial sources, biomass and biofuel burning, and biogenic emissions are significantly larger. The zonal characteristics of TCO derived from satellite measurements are generally simulated by a global chemical transport model called MOZART‐2, but some discrepancies are also shown. The model results are analyzed to delineate the relative importance of surface NOx emission, lightning NOx and stratospheric flux.

Mesospheric temperature inversions over the Indian tropical region

Abstract. To study the mesospheric temperature inversion, daily temperature profiles obtained from the Halogen Occultation Experiment (HALOE) aboard the Upper Atmospheric Research Satellite (UARS) during the period 1991-2001 over the Indian tropical region (0-30° N, 60-100° E) have been analyzed for the altitude range 34-86km. The frequency of occurrence of inversion is found to be 67% over this period, which shows a strong semiannual cycle, with a maximum occurring one month after equinoxes (May and November). Amplitude of inversion is found to be as high as 40K. Variation of monthly mean peak and bottom heights along with amplitude of inversions also show the semiannual cycle. The inversion layer is detected most frequently in the altitude range of 70-85km, with peak height ranging from 80 to 83km and that of the bottom height from 72 to 74km. A comparison of frequency of temperature inversion with that obtained from Rayleigh lidar observations over Gadanki (13.5° N, 60-100° E) is found to be reasonable. The seasonal variation of amplitude and frequency of occurrence of temperature inversion indicates a good correlation with seasonal variation of average ozone concentration over the altitude range of the inversion layer.

Average nighttime F region disturbance neutral winds measured by UARS WINDII: Initial results

We use low‐ and mid‐latitude wind data from the Wind Imaging Interferometer (WINDII) on board the Upper Atmosphere Research Satellite (UARS) to study the average response of nighttime upper thermospheric winds to geomagnetic activity. We calculate perturbation winds in magnetic coordinates and analyze them as a function of magnetic local time, latitude, geomagnetic activity, and solar EUV flux. The nighttime zonal disturbance winds are predominately westward, with the strongest effects extending from dusk at 70° to midnight at 45°. Westward disturbance winds are also observed throughout most of the night at low latitudes, where they change to eastward at dawn. Eastward perturbations occur in the post‐midnight sector above 50°. The meridional disturbance winds are primarily equatorward above 40° and after 0300 MLT. In the midnight sector during low and moderate solar flux conditions, poleward winds are observed below 40°; during solar maximum, the perturbations are largely equatorward throughout the night.

A composite Mg II index spanning from 1978 to 2003

The Mg II core‐to‐wing ratio is a measure of solar chromospheric variability. The Mg II Index, formed by combining various Mg II core‐to‐wing data sets, has been used in EUV, UV, and total solar irradiance models. It is one of the longest records of solar variability reaching back nearly 25 years. We present a single, continuous time series of the Mg II core‐to‐wing ratio extending from November 1978 to the present. The Mg II core‐to‐wing ratio is a measurement that is well suited to the creating of a single time series despite the fact that the seven different instruments measuring the solar flux near 280 nm have different spectral resolutions and sample rates. The Upper Atmosphere Research Satellite (UARS) Solar Ultraviolet Spectral Irradiance Monitor (SUSIM), UARS Solar Stellar Irradiance Comparison Experiment (SOLSTICE), ERS‐2/Global Ozone Monitoring Experiment (GOME) and five NOAA solar backscatter ultraviolet data sets were used. Initially, the best data sets were selected to create a time series spanning from 1978 to the present. Then the gaps in the record were filled with data from various other Mg II data sets. Where no alternate data were available, a cubic spline function was used to bridge the missing data. In some cases the data gaps were too long for reasonable spline fits (more than 5 days), and for these gaps the F10.7 cm flux data were scaled to fill the gaps. Thus a continuous, uninterrupted time series of the Mg II core‐to‐wing ratio was created. The final Mg II time series is compared with other solar activity indices, such as the F10.7, He I 1083, and Sunspot number, to look for trends in the Mg II data.

Stratospheric N2O5 in the austral spring 2002 as retrieved from limb emission spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)

N2O5 was retrieved from infrared limb emission spectral radiances made by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the European environmental satellite (ENVISAT). The measurements were taken during the period covering the Antarctic polar vortex split between 20 September and 13 October 2002. The retrieval of N2O5 is aggravated by its continuum‐like emission feature covering a wide spectral region, which is hardly distinguishable from background continuum emission. The method of constraining the background continuum in the N2O5 analysis spectral region to its value in a N2O5‐free spectral region was found to be appropriate to solve this problem. Retrieved volume mixing ratios (VMR) of N2O5 exhibit features consistent with the dynamics prevalent at the time in the Antarctic and known N2O5 chemistry governing diurnal variability. The observations of low N2O5 inside vortex air mass and rich N2O5 exvortex air mass are strongly in support of the chemistry that governs its partitioning within the NOy family. The enhanced nighttime high geographic latitude N2O5 VMR with a peak of 4.4 ppbv in the altitude range of 32–37 km during the last week of September 2002 is consistent with air mass transport from lower to high latitudes and temperature‐sensitive N2O5 formation chemistry. N2O5 enhancement up to 6 ppbv was also observed by the Cryogenic Limb Array Etalon Spectrometer(CLAES) and Improved Stratospheric and Mesospheric Sounder (ISAMS) experiments on Upper Atmospheric Research Satellite (UARS) during Northern Hemisphere January 1992 stratospheric warming which was a factor of 3 larger than any measurements of N2O5 prior to UARS. In contrast, a maximum of 4.4 ppbv N2O5 VMR observed by MIPAS at 32–37 km is only a factor of 2 larger than its prewarming values.