B. David Green

Experimental determination of the Einstein coefficients for the N2(B–A) transition

We have used a branching‐ratio technique to measure the relative variation in the transition‐dipole moment with internuclear separation for the N2(B–A) transition. Our spectral observations cover the range from 500 to 1800 nm, and use several different detectors and excitation sources. The data from different sets are consistent in the regions of spectral overlap. Using well established values for the radiative lifetimes of N2(B,v’≥5) allows the relative dipole‐moment function to be placed on an absolute basis. From the dipole‐moment function and a set of RKR‐based Franck–Condon factors which we have computed, we derive Einstein coefficients covering the range v’=0–12 and v‘=0–20. Our results indicate that currently accepted lifetimes for N2(B,v’=0–2) should be revised upwards by 20% to 40%.

Analysis of hydroxyl earthlimb airglow emissions: Kinetic model for state‐to‐state dynamics of OH (υ,N)

Detailed spectroscopic analysis of hydroxyl fundamental vibration-rotation and pure rotation emission lines has yielded OH(υ,N) absolute column densities for nighttime earthlimb spectra in the 20 to 110-km tangent height region. High-resolution spectra were obtained in the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) experiment. Rotationally thermalized populations in υ = 1–9 have been derived from the fundamental bands between 2000 and 4000 cm−1. Highly rotationally excited populations with N ≤ 33 ( ≤ 2.3 eV rotational energy) have been inferred from the pure rotation spectra between 400 and 1000 cm−1. These emissions originate in the airglow region near 85–90 km altitude. Spectral fits of the pure rotation lines imply equal populations in the spinrotation states F1 and F2 but a ratio Π(A′):Π(A″) = 1.8±0.3 for the Λ-doublet populations. A forward predicting, first-principles kinetic model has been developed for the resultant OH(υ,N) limb column densities. The kinetic model incorporates a necessary and sufficient number of processes known to generate and quench OH(υ,N) in the mesopause region and includes recently calculated vibration-rotation Einstein coefficients for the high-N levels. The model reproduces both the thermal and the highly rotationally excited OH(υ,N) column densities. The tangent height dependence of the rotationally excited OH(υ,N) column densities is consistent with two possible formation mechanisms: (1) transfer of vibrational to rotational energy induced by collisions with O atoms or (2) direct chemical production via H + O3 → OH(υ,N) + O2.

N2+ Meinel band quenching

We have measured the rate coefficients for quenching the A 2Πu state of N2+ by air to be (7.0±0.4), (7.5±1.0), and (7.0±1.0)×10−10 cm3 molecule−1 s−1 for vibrational levels 2–4, respectively. Rate coefficients for quenching vibrational level 2 by molecular nitrogen and oxygen are (7.5±0.8) and (6.2±0.6)×10−10 cm3 molecule−1 s−1, respectively. Our results show that Meinel‐band quenching becomes significant at altitudes below100 km.

EXTENDED PERFORMANCE HANDHELD AND MOBILE SENSORS FOR REMOTE DETECTION OF NATURAL GAS LEAKS

This report summarizes work performed by Physical Sciences Inc. (PSI) to advance the state-of-the-art of surveying for leaks of natural gas from transmission and distribution pipelines. The principal project goal was to develop means of deploying on an automotive platform an improved version of the handheld laser-based standoff natural gas leak detector previously developed by PSI and known as the Remote Methane Leak Detector or RMLD. A laser beam which interrogates the air for methane is projected from a spinning turret mounted upon a van. As the van travels forward, the laser beam scans an arc to the front and sides of the van so as to survey across streets and to building walls from a moving vehicle. When excess methane is detected within the arc, an alarm is activated. In this project, we built and tested a prototype Mobile RMLD (MRMLD) intended to provide lateral coverage of 10 m and one lateral scan for every meter of forward motion at forward speeds up to 10 m/s. Using advanced detection algorithms developed as part of this project, the early prototype MRMLD, installed on the back of a truck, readily detected simulated gas leaks of 50 liters per hour. As a supplement to the originally planned project, PSI also participated in a DoE demonstration of several gas leak detection systems at the Rocky Mountain Oilfield Testing Center (RMOTC) during September 2004. Using a handheld RMLD upgraded with the advanced detection algorithms developed in this project, from within a moving vehicle we readily detected leaks created along the 7.4 mile route of a virtual gas transmission pipeline.

Flashlamp measurement of the MSX particulate environment

The xenon flashlamp is one of a suite of instruments that monitor the particulate and gaseous contamination environments of the midcourse space experiment (MSX) spacecraft. The near-field particulate measurement comprises the high intensity xenon flashlamp that illuminates a volume of space in the field of view of the UVISI wide field of view visible imager (UVISI IVW). Radiation scattered by illuminated contaminant particles is imaged by the IVW. The intensity of the radiation is related to a particles size and composition. The particles track yields information about its velocity and trajectory. From ground calibration data we estimate a sensitivity to detect particles smaller than 1 micrometer and to determine cross-field velocities from 1 mm/sec to 50 m/sec. The visible radiation measurement of the particulate environment provided by the xenon flashlamp and UVISI IVW is complemented by multiband IR, UV, and visible measurements by other MSX sensors. The early mission data from this experiment will quantify the relationship between ground contamination control measures, the on-orbit contamination environment, and the performance history of on-orbit sensors.

Long-term observations of the particle environment surrounding the MSX spacecraft

We present a summary of the particle environment surrounding the Midcourse Space Experiment (MSX) satellite after 32 months on orbit, including two discrete particle releases produced by micrometeoroid or debris impact. We report on the characteristics of that environment, including particle occurrence rates, velocities, size distributions and trends in the environment. To our knowledge, the long term particle contamination observations that we have made on MSX are the first of their kind. The particle occurrence rate decreased steadily during the first year on orbit, but then remained at a constant level after 32 months on orbit. Our estimate of the total number of particles on the spacecraft surfaces at launch. We conclude that environmental effects such as UV, radiation, thermal cycling, and micrometeoroid impacts are a significant and continuing source of particles on orbit.

Measurement of long-term outgassing from the materials used on the MSX spacecraft

The Midcourse Space Experiment (MSX) spacecraft was specifically designed and processed to minimize contamination. This spacecraft represents a best case scenario of spacecraft induced environment. The contamination instrument suite consisted of 10 sensors for monitoring the gaseous and particulate environment. The Total Pressure Sensor (TPS) has continuously measured the ambient local pressure surrounding MSX since its launch on April 24, 1996. The sensors primary goal was to monitor the early mission (less than one week) ambient pressure surrounding the spacecrafts optical telescopes and to indicate when environmental conditions were acceptable for opening the protective covers. However, the instrument has illustrated that it is quite robust and has successfully measured the long-term decay of the pressure environment. The primary constituent of the atmosphere is water outgassed from the thermal blankets of the spacecraft. The water-induced environment was expected to rapidly decay over the first few months to levels more closely approaching the natural environment. The data generally shows decay toward this level, however, the pressure is quite variable with time and can be influenced by discrete illumination and spacecraft orbital events. Several experiments conducted yearly indicate that the thermal blankets retain significant quantities of water. The local pressure due to water vapor is shown to increase by a factor of 100 from direct solar illumination. Moreover, the multi-layer construction of the blankets causes them to form a deep reservoir that continues to be a source of water vapor 3+ years into the mission. We will present pressure data from several experiments, each separated by one orbital year, that exhibit these water vapor induced pressure busts. The decay and longevity of these bursts will also be discussed.

Laser cleaning of cryogenic optics

IR laser heating is the subject of tests to determine its effectiveness as a method of removing optical contaminants with special attention given to on-orbit decontamination. A variety of mirrors are contaminated in a vacuum at a temperature of about 100 K with a BRDF diagnostic monitoring used to measure cleanliness before and after contamination and after laser cleaning. Laser treatments with CO2 and Nd:YAG lasers are investigated for contaminants such as H2O, CO2, and dust. The contaminants usually degraded the BRDF by a factor of about 2, and the laser treatments are generally able to return the BRDF to the precontamination level. The Nd:YAG laser treatment relies on heating the mirror surface and is not as effective and applicable as that of the CO2 laser. Successful cleaning can be achieved at temperatures of 35-300 K with thick contaminant films without damaging or distorting the mirror surface.

Update of the midcourse space experiment (MSX) satellite measurements of contaminant films using QCMs

The Midcourse Space Experiment (MSX) satellite was launched on April 24, 1996. This paper provides an update of the quartz crystal microbalance (QCM) data accumulated over these last four years in space. The MSX is the only known experiment that has provided continuous contamination monitoring for such an extended length of time. The five QCMs on board the satellite have provided on-orbit data that have been invaluable in characterizing contamination levels around the spacecraft and inside the cryogenic Spatial Infrared Imaging Telescope (SPIRIT 3). One of the QCMs, the cryogenic QCM (CQCM), located internal to SPIRIT 3, was mounted adjacent to the primary mirror and provided contamination accretion measurements during the 10-month lifetime of SPIRIT 3. Real- time monitoring of contaminant mass deposition on the primary mirror was provided by this CQCM which was cooled to the same temperature as the mirror - approximately 20K. Thermogravimetric analyses (TGAs) on the CQCM provided insight into the amount and species of contaminants condensed on the SPIRIT 3 primary mirror during various spacecraft activities. The four temperature-controlled QCMs (TQCMs) were mounted on external surfaces of the spacecraft for monitoring spacecraft contamination deposition. The TQCMs operated at approximately -50

Satellite contamination and materials outgassing effects databases

DEGC and were positioned strategically to monitor the silicone and organic contaminant flux arriving at specific locations. Updated time histories of contaminant thickness deposition for each of the QCMs are presented. Gradual contaminant thickness increase was observed during the first year in space. During the second year, the QCM frequencies (contaminant film thickness) began to decrease, with the time of onset depending on QCM location. Possible explanationsfor this interesting behavior are discussed.