SA21A-0332 0800h
The Effect of Diurnal Variations on Decadal Trends in PMC Brightness
The Solar Backscatter Ultraviolet (SBUV and SBUV/2) instruments on the NOAA polar-orbiting meteorological satellites have shown that there has been an increase in the average brightness of Polar Mesospheric Clouds (PMC) since 1978 [Thomas et al. 2003]. However, the equator-crossing times of most different satellite platforms have drifted over the life of the satellite, resulting in corresponding variations in the local time of the PMC observations. Measurements from the WINDII instrument on UARS [Shettle et al. 2002], as well several lidar groups have reported diurnal variations in the PMC brightness. This raises the possibility that systematic changes in the local time of the PMC measurements SBUV over the past 25 years have contributed to the reported brightness trends. Each SBUV instrument provides PMC observations at different local times on the ascending and descending nodes of the satellite, and with two or three different instruments in orbit over most of this period there are up to six different local times of measurement on a given day (for each latitude band). We can use these multiple daily measurements to separate the effects of any systematic changes in the local time of the measurements from any long-term trends. Preliminary results indicate that there is still a multi-decadal increase in the average brightness of the PMCs independent of any systematic changes in the local time of the measurements.
SA21A-0333 0800h
Spectral Measurements of PMCs from SBUV/2 Instruments
The SBUV/2 instrument is designed to monitor stratospheric profile and total column ozone using measurements of the Earth's backscattered ultraviolet albedo. We have previously demonstrated that the normal radiance measurements from SBUV/2 instruments, which sample 12 discrete wavelengths between 252-340 nm during each scan, can be used to identify polar mesospheric clouds (PMCs). A 25-year PMC data set has been constructed using data from six SBUV/2 instruments, three of which are currently operating. Some SBUV/2 instruments also periodically view the Earth in continuous scan mode, covering the wavelength range 160-400 nm with 0.15 nm sampling. These data can be analyzed for the presence of PMCs using the same detection algorithm applied to discrete SBUV/2 data. PMC spectra observed by SBUV/2 instruments show a monotonic variation in brightness over the wavelength range 250-295 nm, with maximum enhancements above the background albedo of 10-15% at 250 nm. This result is consistent with microphysical model predictions, using a mean particle radius of 60 nm and a log-normal distribution width of 1.4. We find no evidence for a systematic localized increase in PMC brightness near 260 nm, in contrast to the recently reported results from the MSX UVISI instrument [{\it Carbary et al.}, 2004]. The flat spectral dependence of PMCs in the mid-UV is observed from three different SBUV/2 instruments in both Northern and Southern Hemisphere data over a 13-year span. The spectral signature of the SBUV/2 continuous scan PMC data does not require the existence of a bimodal particle size distribution, as suggested by {\it Carbary et al.} [2004]. Carbary, J. F., D. Morrison, and G. J. Romick (2004), Evidence for bimodal particle distribution from the spectra of polar mesospheric clouds, {\it Geophys. Res. Lett.}, {\it 31}, L13108, doi:10.1029/2004GL020101.
SA21A-0334 0800h
Vertical structure of NLCs observed by Odin/OSIRIS
The vertical variation of NLC particle sizes is investigated with limb scattering observations in the UV-B spectral range performed with the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite. The particle sizes are determined under the assumptions that NLC particles can be treated as Mie-scatterers with known refractive index, and that their size distribution can be approximated by a log-normal distribution. OSIRIS is particularly suited to resolve vertical NLC structure due to its narrow vertical field of view of about 1 km. A sample data set of several dozen bright NLCs detected on different days during the 2003 NLC season in the northern hemisphere and the 2003/2004 NLC season in the southern hemisphere was selected for this study. In almost all the cases the derived NLC particle size increases with decreasing altitude. This confirms the standard picture of larger particles at the bottom of the cloud due to sedimentation. The derived particle mode radii estimates fall within the range from about 20 to 60 nm assuming a width parameter of $\sigma=1.4$ for the log-normal distribution.
SA21A-0335 0800h
Some Prominent Temporal Variations in the Metal Layers in the Upper Atmosphere.
Using resonant radiation in the 237-300 nm range measured by the nadir viewing Global Ozone Measuring Experiment (GOME) UV/VIS spectrometer on the ERS-2 satellite, temporal changes in the vertical column contents of the most dominant meteoric metal species Mg$^{+}$, Mg, Fe$^{+}$, Fe and Si have been analyzed in November, encompassing Leonid meteor shower period. Several years of data are available for analysis. ERS-2 orbits the Earth 14 times per day at 795 km in a sun synchronous orbit with an equatorial crossing time of 10:30 AM. For this study data were restricted to the latitude zone of 10$^{o}$N to 30$^{o}$N, straddling the declination of the shower radiant. The presence of a Leonid shower effect in the studied measurements is not clear cut in 1996. During one of the visible shower periods associated with the 1996 Leonid, there appears to be an ion-neutral composition change associated with one of the showers - i.e., the ion content increases during the maximum of the shower activity and it precedes an enhancement of the neutral species. However during the entire month there are many similar enhancements unassociated with the shower. Also, it is found that there are longitudinal effects in the measurements, most notably in the measured neutral Si, which is observed to have surprisingly high vertical contents. A detailed comparison is made between the variations observed in 2000 and ground-based observations of the shower activity - this was a particularly well-surveyed event.
SA21A-0336 0800h
Investigation of Mesospheric Metal Emission Signals From SCIAMACHY Limb Measurements
First preliminary results of global UV dayglow measurements of metallic species like Mg, Fe and Na as well as their ionic counterparts, and Si and Si+ are presented. These measurements are currently undertaken by the SCIAMACHY instrument operated onboard the European research satellite ENVISAT. SCIAMACHY covers the wavelength range from 220 to 2380 nm with a spectral resolution of 0.2 to 1.5 nm and provides nadir, limb and occultation viewing modes. The metal signals observed occur in the wavelength range 220-310 nm. The limb viewing mode covers altitude levels from 0 to 100 km, whereas the nadir mode provides total columns ranging from ground to the exosphere; however, in the wavelength region of metallic emissions, due to atmospheric opacity no information is contained in measurements below ~ 60 km. Vertical profiles of concentrations are obtained from Limb measurements by modelling the excitation process for all limb levels above ~ 60km. Regarding metallic species, very few other measurements exist. Here, we report the first measurements of altitude profiles of Mg, Mg+, Si and Si+ made by a satellite-borne instrument.
http://www.iup.physik.uni-bremen.de/~miriam/sciametals.html
SA21A-0337 0800h
Observations of Ultraviolet Emission from Mg$^+$ in the Lower and Middle Thermosphere
New observations of ionized magnesium dayglow are reported from the Ionospheric Spectroscopy and Atmospheric Chemistry (ISAAC) instrument on the ARGOS satellite. We focused on two periods, October 14-28 1999 and November 15-30 1999, when ISAAC obtained high quality limb spectra between 2600 and 3000 \AA\ and from 85 to 350 km tangent altitude. In addition to the resonant scattering by Mg$^+$ near 2800 \AA, these limb spectra also contain signatures of fluorescent scattering by nitric oxide in the gamma bands, emission by molecular nitrogen in the Vergard-Kaplan bands, and atomic emission by oxygen in the 2972 \AA\ line. A retrieval algorithm has been developed to measure the abundance of nitric oxide using the intensity of fluorescent scattering in the $\gamma$ (1,5) band at 2670 \AA. This technique then allows for separating the overlapping emission by nitric oxide in the $\gamma$ (1,6) band from the Mg$^+$ doublet at 2800 \AA. Retrieved Mg$^+$ column densities have been mapped as a function of altitude and geomagnetic latitude.
SA21A-0338 0800h
Electron Density Retrieval Validation Using 911\AA Emission Limb Scans
This study will focus on a statistical validation of a one-dimensional electron density retrieval algorithm for the oxygen emission at 911\AA. To do this, we will use data from ultraviolet limb scans to perform electron density profile retrievals using a one-dimensional algorithm. We will be analyzing the peak density of the layer as well as the height of the maximum density. These findings will be compared with data from ground-based ionosondes. The data set will cover a period of several months. Our particular interest is in data from limb scans near the terminator, though we will also analyze dayside 911\AA emission data. The results of this study will be valuable for the Special Sensor Ultraviolet Limb Imager (SSULI) on a recently launched satellite of the Defense Meteorological Satellite Program (DMSP). This first satellite to carry a SSULI instrument was launched October 18, 2003 into an orbit close to the terminator. This study will also be valuable for the next DMSP satellite to carry a SSULI instrument, which will fly in a terminator orbit. The SSULI instruments are similar to their prototype, the Low Resolution Airglow and Auroral Spectrograph (LORAAS), which was operational between 1999-2001. The LORAAS measured upper atmospheric airglow in the far- and extreme-ultraviolet passband, producing one limb scan every ninety seconds. We plan to use this analysis to demonstrate the value of using the 911\AA emission for the retrieval of electron density profiles on the dayside and in the terminator.
SA21A-0339 0800h
Measurements of Rotational Line f-Values of N2 in the 83.4 and 91.7 nm Regions: High-Resolution, High-Temperature
We report the temperature-dependent ultrahigh-resolution photoabsorption spectra of N2 and O2 in the extreme ultraviolet (EUV) region, and their atmospheric modeling implications. One of the important issues regarding the interpretation of the NII 91.6 nm extreme ultraviolet airglow emissions of the Earth, Titan, and Triton, and the OII 83.4 nm EUV airglow emission of the Earth, is the effect of temperature on the atmospheric extinction due to absorption by N2 and O2. Since the temperature of the upper atmosphere of the Earth is typically in the 200 to 900 K range it is therefore important to know these molecular cross sections at such temperatures. We have carried out high resolution photoabsorption cross section measurements of N2 with a resolution of 0.0003 nm and 0.0008 nm in the 91.5-91.728 nm and the 83.31-83.45 nm regions at temperatures of 600, 535 and 295 K. The 6VOPE (6.65-m vertical off-plane Eagle spectrograph) spectrometer available at the Photon Factory, KEK, Tsukuba, Japan, was employed in the present study. The N2 absorption features in the 83.4 nm region mainly involve the (0,0) band of the c'6 - X transition and a weak (2,0) band of the c'4 - X transition. The rotational assignments have previously been given [Carroll and Yoshino, 1972]. Spectral perturbation effects in the c'6 - X transition have been observed at J (rotational quantum number) > 17. The weak (2,0) band of the c'4 - X transition further contributes to the spectral complexity. By integration over each individual absorption profile the rotational line f-values of N2 are determined for the above-mentioned spectral regions and experimental conditions. The detailed results will be presented. This research is based on work supported by NSF grant ATM-0096761.
SA21A-0340 0800h
Nightglow Spectral Lines: The HIRES and UVES Database
The terrestrial nightglow, from the ozone cut-off near 310 nm to the optical IR limit near 1050 nm, contains several thousand emission lines. A high-resolution atlas of 2800 lines has recently been published,[\textit{Hanuschik}, 2003] collected with the UVES echelle spectrograph on the VLT Kueyen 8 m telescope at the European Southern Observatory Paranal site in Chile. No attempt was made to assign the lines. Using similar data from the HIRES echelle spectrograph on the Keck I telescope, we have assigned most of the lines appearing in the UVES atlas. In this presentation we compare the quality of the UVES and HIRES data, finding them to be comparable at wavelengths longer than 340 nm, with better data from UVES below that wavelength. The line positions compare very well, with average deviations in the UV on the order of 0.0007 nm. The new O$_2$(\textit{c}$^1\Sigma_u^-$--\textit{b}$^1\Sigma_g^+$) emission system,[\textit{Slanger et al.}, 2003] is prominent in both sets of spectra, and we demonstrate that many of its lines were measured almost half a century ago.[\textit{Chamberlain}, 1958] This work was supported by NSF Aeronomy and NSF Astronomy Chamberlain, J.W., The Blue Airglow Spectrum, \textit{Astrophys. J. 128}, 713--717, 1958. Hanuschik, R.W., A Flux-Calibrated High-Resolution Atlas of Optical Sky Emission from UVES, \textit{Astron. Astrophys. 407}, 1157--1164, 2003. Slanger, T.G., P.C. Cosby, and D.L. Huestis, A New O$_2$ Band System: The \textit{c}$^1\Sigma_u^-$--\textit{b}$^1\Sigma_g^+$ Transition in the Terrestrial Nightglow, \textit{J. Geophys. Res. 108} (A2), 1089, 2003.
SA21A-0341 0800h
UV Limb Scan and X-Ray Occultation Soundings of Thermospheric Density
The Naval Research Laboratory flew the Unconventional Stellar Aspect (USA) and High Resolution Airglow and Aurora Spectrograph (HIRAAS) experiments aboard the Advanced Research and Global Observation Satellite (ARGOS) during 1999--2002. USA was a collimated proportional counter X-ray telescope with a large collecting area and microsecond time resolution for 1--10 keV astronomical observations. Additionally, by acquiring occultation spectra of celestial objects USA measured total atmospheric density in the Earth's mesopause and lower thermosphere regions (80--130~km). HIRAAS was a suite of three middle-, far- and extreme-ultraviolet spectrographs for performing limb scan measurements of the Earth's thermosphere and ionosphere (100--750~km). HIRAAS and USA operated simultaneously and were mounted on the same side of the ARGOS spacecraft, enabling novel UV and X-ray studies of the Earth's atmosphere. We present atmospheric density retrievals using simultaneous X-ray and UV measurements from 2000 and compare the results with climatological thermosphere models. The consistency, complementarity, and accuracy of the datasets are discussed, along with relevant space weather considerations, such as the effects of solar X-ray flares upon lower thermosphere structure.
SA21A-0342 0800h
Remote sensing of O, N$_{2}$, and NO in the mesosphere and lower thermosphere using spatial heterodyne spectroscopy
The composition and temperature of the mesosphere and lower thermosphere can be obtained by inverting limb profiles of middle UV airglow emissions near 214~nm and 247~nm. At high altitudes (above 200 km), emissions related to O and N$_{2}$ are prominent. Below 90~km, emissions from NO dominate the scene. In between these regions a merged spectrum of emissions also represents an interaction point for physical processes between the mesosphere and thermosphere. However, the overlap of these spectral features creates a hurdle to obtaining the high fidelity data needed to retrieve atmospheric parameters. Uncertainties in emission sources are introduced when spectral features cannot be distinguished and modeling is required to fit the data. Spatial heterodyne spectroscopy (SHS) has the capability of obtaining high spectral resolution (0.01~nm) measurements of these prominent middle UV emissions from the mesosphere and lower thermosphere. This spectral resolution would eliminate much of the uncertainty inherent in fitting the emission feature, thereby improving the accuracy of the emission profile. We present simulations from an SHS instrument being investigated for development at the Naval Research Laboratory to make measurements at 214~nm and 247~nm. Instrument requirements are derived from existing lower resolution spectra and profiles, the inversion of simulated data, and the desired accuracy of the retrieved parameters. Our results show the enticing capability for remote sensing of this under-explored region using SHS.
SA21A-0343 0800h
Airglow Observation from STSAT-1
STSAT-1, also known as KAISTSAT-4, has been in operation more than a year since its launch on September 26, 2003. While the primary objective of the mission is to provide spectral information of diffuse hot Galactic plasmas and supernova remnants with Far Ultra-violet Imaging Spectrograph (FIMS, also known as SPEAR), the same instrument also makes observations of auroras and airglow with high spatial resolution and detailed spectral information for a space physics mission. The FIMS bandwidth, 900 - 1175 A and 1335 - 1750 A, includes important oxygen and nitrogen lines such as OI 989 A, N+ 1085 A, N I 1134 A and 1493 A, as well as Lyman-Birge-Hopfield (LBH) emissions. Over the polar region, FIMS spectral image is compared with simultaneous in-situ measurements of keV electrons on the same spacecraft. FIMS observes auroras regularly twice a day and airglow occasionally. Especially, FIMS made limb scanning on several occasions in the dayside using its short band (900 - 1175 A). We will discuss the height profile of atomic oxygen and nitrogen obtained from this set of observations. We will also compare the results with those of AURIC simulations.
SA21A-0344 0800h
Comparison of the Ultraviolet Aurora at High and Low Spatial Resolution
On 22 and 23 November 1997, ultraviolet imagers and spectrographic imagers on the Midcourse Space Experiment (MSX) observed the aurora at sub-kilometer spatial resolution at wavelengths from 114 nm to 385 nm. At the same time, the Polar Ultraviolet Imager (UVI) observed the aurora at similar wavelengths, but at spatial resolutions two orders of magnitude larger. UVI measurements in certain far-UV bands are often used to determine auroral energy deposition, characteristic energy, and composition, but only on a gross spatial scale. For two polar passes in November, this paper compares key auroral radiances (130.4 nm, 135.6 nm, and the LBHS and LBHL bands) at high spatial resolution from MSX with those at low spatial resolution from UVI. As might be expected, the high resolution data exhibit considerably more structure than the low resolution data, both spatially and temporally. Degradation of the high spatial resolution radiances to low resolution will determine if the latter accurately can represent auroral energetics and composition.