P. W. Erdman

Carbon Dioxide and Oxygen Isotope Anomalies in the Mesosphere and Stratosphere

Isotopic (δ17O and δ18O) measurements of stratospheric and mesospheric carbon dioxide (CO2) and oxygen (O2), along with trace species concentrations (N2O, CO, and CO2), were made in samples collected from a rocket-borne cryogenic whole air sampler. A large mass-independent isotopic anomaly was observed in CO2, which may in part derive from photochemical coupling to ozone (O3). The data also require an additional isotopic fractionation process, which is presently unidentified. Mesospheric O2 isotope ratios differed from those in the troposphere and stratosphere. The cause of this isotopic variation in O2 is presently unknown. The inability to account for these observations represents a fundamental gap in the understanding of the O2 chemistry in the stratosphere and mesosphere.

Validation of Halogen Occultation Experiment CH4 measurements from the UARS

Global distributions of CH4 in the mesosphere and stratosphere have been measured continuously since October 11, 1991, by the Halogen Occultation Experiment (HALOE) onboard the UARS. CH4 mixing ratio is obtained using the gas filter correlation technique operating in the 3.3-μm region. Since measurements are made during solar occultation in the 57° inclination orbit, data are collected 15 times daily for both sunrises and sunsets. This provides coverage of one hemisphere in a month period. One complete hemispheric sweep (from equator to ∼80° latitude) is made during the spring and summer seasons of two hemispheres, and a partial sweep (from equator to around 50° latitude) is made during the fall and winter seasons of two hemispheres. HALOE CH4 measurements are validated using direct comparisons with correlative data and internal consistency checks using other HALOE-measured tracers, HF, and aerosols. It is estimated for the 0.3- to 50-mbar region that the total error, including systematic and random components, is less than 15% and that the precision is better than 7%. The CH4 gas filter channel does not depend significantly on the Pinatubo aerosol extinction. An experimentally accurate measurement of CH4 is very important because CH4 is a primary interfering gas in the HALOE HCl channel and, subsequently, can cause HCl measurement error. Simultaneous measurements of CH4 and other HALOE species (O3, H2O, NO, NO2, HCl, HF, and aerosol extinction coefficients) provide important information on atmospheric dynamic and chemical processes, since CH4 can be used as a tracer and an indicator of atmospheric transport processes. Several new pieces of information on previously unreported HALOE-observed features are also presented.

Low‐voltage, high‐current electron gun

A simple, high‐performance, electrostatically focused electron gun, which is useful over an energy range extending from a few electron volts to at least 500 eV, is described. The current is delivered in a beam ∼1 mm in diameter, with <50 mrad angular divergence and typically has a current over 1 mA at 100 eV. The design is small, compact, and rugged. This electron gun has been used successfully in both laboratory and sounding rocket experiments.

Measurements of ultraviolet radiation from a 5-km/s bow shock

Ultraviolet emission from a 5.1-km/s re-entry bow shock was measured in a sounding rocket experiment launched from the Barking Sands Research Range (Kauai, Hawaii) in February 1991 at 14:30 GMT. Optical data were obtained on the downleg portion of the flight as the pay load descended from 115 to 62 km in a very shallow trajectory at a nearly constant speed. The intensity of the ultraviolet spectrum (A200-400 nm), and the vacuum ultraviolet resonance radiation emitted by atomic oxygen and hydrogen at A130.4 nm and A121.5 nm, respectively, were measured. Data from optical instruments in the 200-400-nm spectral region is presented here. Langmuir probe measurements provided data on the total plasma density and electron temperature in the boundary layer over a limited altitude range.

Comparison of theory with experiment for the bow shock ultraviolet rocket flight

Comparison is made between the results obtained from a state-of-the-art thermochemical nonequilibrium flowfield and radiation code and data obtained from a recent experiment. The experiment obtained the first measurements of ultraviolet radiation from the shock-heated gas in the nose region of a 0.1016-m nose radius vehicle traveling at about 3.5 km/s at altitudes between 37-75 km. The preflight computations agree at low altitudes but underpredict the data at high altitudes. Postflight flowfield and radiation sensitivity studies suggest improvements for the models at high altitudes. Specifically, excitation mechanisms that contribute to production of NO gamma-band emission need to be revised. Altitude dependence of the radiation observed from the OH radical can be understood in terms of nonequilibrium chemistry in the flow.

Validation of measurements of carbon monoxide from the improved stratospheric and mesospheric sounder

Carbon monoxide abundances in the middle atmosphere are retrieved from infrared measurements of the emission of this molecule at 4.6 μm taken by the improved stratospheric and mesospheric sounder (ISAMS) on board the Upper Atmosphere Research Satellite. In addition to a limited signal-to-noise ratio, the measured radiances include significant contamination by other compounds in the stratosphere and show clear non-local thermodynamic equilibrium effects in the mesosphere which also affect the stratospheric limb views. The application of the ISAMS retrieval method to these measurements and the particular method followed to deal with these difficulties are described. The actions carried out to understand and validate the obtained CO abundances are detailed. They include analysis of the radiances, sensitivity studies of the product to retrieval parameters, evaluation of the main sources of systematic and random errors, and comparisons with theoretical model predictions and with the few correlative measurements available. The gross features observed in the CO distribution present good agreement with dynamical-chemical models of the middle atmosphere. The aspects of the retrieval that need further improvement are identified, and the global quality of the database and the expectations for its scientific use are discussed.

Flight measurements of low-velocity bow shock ultraviolet radiation

The ultraviolet spectrum, atomic oxygen 130.4-nm radiation intensity, total plasma density, and electron temperature of a Mach 12 bow shock were obtained by a sounding rocket experiment launched from the Wallops Flight Facility (WFF) on April 25, 1990 at 12:32 a.m. Eastern Standard Time (EST). A two-stage, Terrier Malamute rocket which attained an apogee of 720 km was used in this experiment. Optical data in the 200400-nm wavelength range were obtained from 37 to 75 km at a vehicle velocity of 3.5 km/s at various locations on the 0.1016-m radius hemispherical dome. Electron probe and VUV OI 130.4-nm measurements were obtained near nose cone ejection at 37 km. This article presents a discussion of the instruments used and the key data obtained.

Electron-impact excitation of the Cameron system (a3π → X1Σ)of CO

Abstract We have studied the excitation of the Cameron bands of carbon monoxide (a3π → X1Σ+) by electron impact on CO and CO2. This investigation was prompted by a recent study of the Martian airglow by Conway (1981) who concluded that the cross section for the dissociative excitation of the Cameron bands is seven times larger than the laboratory value reported by Ajello (1971a) and by a perplexing inconsistency between the optical cross section and CO(a3π) time-of-flight experiments. We have found now that three factors have contributed to these discrepancies: (1) spectral contamination of the (1,4) Cameron band used by Ajello to normalize the entire Cameron band cross section, (2) major revisions in the magnitude of the CO(a3π) radiative lifetime, and (3) new insights into the effects of the CO(a3π) velocity distribution on the field of view of the emission experiments. The new results largely reconcile the TOF and emission measurements, but they also suggest that the calculated photoelectron fluxes in the Martian atmosphere may be too large by a factor of 3.

Measurements of methane and nitrous oxide distributions by the improved stratospheric and mesospheric sounder: Retrieval and validation

The improved stratospheric and mesospheric sounder (ISAMS) used the pressure modulation technique to observe infrared emissions from the atmosphere at 7.4 μm and 7.8 μm. The target gases, methane and nitrous oxide, both emit at these wavelengths and so their concentrations have been determined by joint retrieval from the available signals. This paper describes the measurement technique, the current retrieval scheme (version 10), and the data set produced. The version 10 data set comprises typically 2600 profiles per day on 180 days between September 26, 1991, and July 29, 1992. Retrieved profiles extend in altitude from 7 mbar to 0.08 mbar for methane and from 7 mbar to 0.8 mbar for nitrous oxide. The precision of the data is better than 20% over much of the range and estimated systematic uncertainties are less than 30%. Comparisons with coincident measurements show that the systematic uncertainties are a reasonable estimate for the methane data but reveal a much larger positive bias for the nitrous oxide data relative to other measurements. Accounting for aerosol contamination effects and a priori biases, ISAMS methane data are recommended for use in scientific studies at altitudes between 7 mbar (5 mbar in the tropics) and 0.1 mbar. ISAMS nitrous oxide data are recommended for use between 7 mbar and 1.0 mbar where relative rather than absolute values are required.

Examination of OH ultraviolet radiation from shock-heated air

Two recent sounding rocket experiments obtained spectral data at wavelengths of 200-400 nm from the shock-heated air surrounding a vehicle under flight conditions of 3.5 km/s at altitudes of 40-70 km and 5 km/s for altitudes of 110-65 km. Previous analyses of the data have emphasized modeling the radiation from the NO molecular system. The chemical kinetics of OH, a trace species in the flow, and the electronic state excitation mechanisms are simpler than those for NO. Hence, the comparison between modeling and data potentially provides a clearer assessment of the modeling of the flow thermochemical processes. This article discusses the OH flowfield and radiation models that we have developed, comparisons with data, and the implications of this work to ongoing NO flow and radiation modeling. Additional data from the side-viewing spectrometers from the 5-km/s flight will also be presented and analyzed. The angular dependence of the photometer data was compared with a solution obtained from a viscous threedimensional flowfield calculation with reacting water chemistry.