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The MSX/UVISI Stellar Occultation Experiments: Proof-Of-Concept Demonstration of a New Approach to Remote Sensing of Earth’S Atmosphere

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The MSX/UVISI Stellar Occultation Experiments: Proof-Of-Concept Demonstration of a New Approach to Remote Sensing of Earth’S Atmosphere The MSX/UVISI Stellar Occultation Experiments: Proof-of-Concept Demonstration of a New Approach to Remote Sensing of Earth’s Atmosphere Ronald J. Vervack Jr., Jeng-Hwa Yee, William H. Swartz, Robert DeMajistre, and Larry J. Paxton primary means of monitoring the atmosphere on a global scale is remote sensing of Earth from space, and occul- tation methods based on observing changes in a star’s spectrum as it sets through the atmosphere have proven valuable in this endeavor. Two occultation techniques—focused respectively on atmospheric extinction and refrac- tion—have been treated separately historically, but the combination of the two offers the possibility of higher accuracy, greater altitude coverage, and simultaneous measure- ment of primary and trace gases, both of which are important in the chemical balance of the atmosphere. Using data from the Ultraviolet and Visible Imagers and Spectrographic Imagers (UVISI) on the Midcourse Space Experiment (MSX) satellite, we have developed a combined extinctive/refractive stellar occultation technique that relates the changes in measured stellar intensity to the height-dependent density profiles of atmospheric con- stituents and have demonstrated its viability for retrieving both primary and trace gases through the analysis of approximately 200 stellar occultations. This self-calibrating tech- nique is broadly applicable and suitable for measurement of gases that are otherwise difficult to quantify on a global basis. In this article, we summarize the development of the technique, its validation through comparison to other measurements, and its applica- tion to the study of Earth’s overall atmosphere and lower atmosphere in particular. INTRODUCTION The Earth’s atmospheric composition, temperature, shorter timescales, however, anthropogenic activi- and ability to cleanse itself chemically have coevolved ties have effected changes in the atmosphere. Much with biological life on geological timescales.1 On much of the observed change is associated with minor atmo- JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 32, NUMBER 5 (2014) 803 VERVACK ET AL. spheric constituents, referred to as trace gases, and their instrument degradation and changes in calibration than impact on atmospheric chemistry. Although many of other remote sensing methods. Although these tech- the Earth’s trace gases exist naturally, the rapid rise in niques are easy to implement in principle, technical the concentrations of both native and nonnative gases hurdles exist. As these challenges have been overcome, and their observed effects on the atmosphere’s chemical occultation techniques have proven to be an excellent balance over the past 150 years or so is coincident with means of exploring the overall structure of both terres- industrialization and the advent of modern agricultural trial and planetary atmospheres. practices.2 Whereas studies assessing the long-term con- Using occultation methods to probe Earth’s lower sequences of these changes are ongoing, a significant atmosphere is complicated by the effects of both obstacle is often a lack of regular, global measurements refraction and extinction as light passes through the of the concentrations of these trace gases in the atmo- densest part of the atmosphere. To demonstrate a new sphere, particularly in the lower atmosphere (less than approach to this challenge, we took advantage of the 50 km altitude) where their concentrations and effects Ultraviolet and Visible Imagers and Spectrographic are most significant. Measurement of emission spectra Imagers (UVISI) suite of instruments onboard the from atoms and molecules is often used to probe atmo- Midcourse Space Experiment (MSX) spacecraft to spheric densities, but many of the important trace gases conduct stellar occultation experiments in which we in the atmosphere emit such radiation weakly or not combined techniques suited to measuring refraction and at all, or their emission signatures are convolved with extinction independently into a single, self-consistent stronger emission from other atoms or molecules and occultation method that leverages the benefits of both cannot be separated. As such, these trace gases can only techniques. This demonstration led to the development be detected through in situ sampling or via remote obser- of an instrument concept suitable for flight on a small vation of their attenuation or scattering of light. space platform. Stellar occultation is one of the most promising tech- The potential measurement capabilities of this com- niques to meet the needs of atmospheric scientists, policy bined stellar occultation technique are illustrated in makers, and enforcement agencies tasked with under- Fig. 1. The left panel shows the retrievable altitude ranges standing and regulating man-made contributions to the for several gas and aerosol constituents in the terrestrial trace gas atmospheric budget. Occultation techniques atmosphere that can be measured over far-UV to visible have been used for many years to study the atmospheres wavelengths, whereas the right panel shows the altitude of Earth, other planets, and their satellites. Because these ranges for gases that can be measured in the near-IR. techniques are generally based on relative measurements These altitude ranges are derived from an analysis of (i.e., compared to measurements in the absence of the the extinction spectra for the various species (a term occulting atmosphere), they tend to be less sensitive to commonly applied to atmospheric constituents) and the 250 60 O2 Wavelength range 100–900 nm Wavelength range CO O2 Generally retrievable 50 2 0.8–2.5 ␮m 200 Under certain conditions 0.8–5.0 ␮m Thermosphere 40 H O CH4 150 2 N2O Aerosols 30 O 3 O2 100 Total 20 Altitude (km) (N2,O2) OCIO Mesosphere Altitude (km) CO NO3 50 NO2 SO2 10 H2O Stratosphere 0 0 Troposphere Figure 1. Illustration of the measurement capabilities of the combined occultation technique. The left panel shows the retrievable alti- tude ranges for several species in the terrestrial atmosphere that can be measured over the wavelength range of 100–900 nm. The blue bars represent ranges that are generally measurable under all conditions, whereas the red bars represent ranges that vary depending on the atmospheric conditions (e.g., night/day, presence of high-altitude clouds) and the target star involved (e.g., spectral type, magni- tude). Colored bars along the right y axis denote the altitude ranges of various atmospheric regions. The right panel shows the retriev- able altitude ranges for several terrestrial species that can be measured if coverage is extended into near-IR wavelengths: 0.8–2.5 m (blue bars) or 0.8–5.0 m (blue+red bars). In both panels, the actual lower limits of the altitude ranges are determined by cloud-top heights at the time of the occultation; the limits shown are for clear skies. Note that the upper limits on CO2 and O3 in the right panel are truncated to 50 km to allow the panel to focus on the lower altitudes. 804 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 32, NUMBER 5 (2014) MSX/UVISI STELLAR OCCULTATION EXPERIMENTS capabilities of an instrument optimized for their mea- mately 50 remain suitable out to 2.5 m, and only 10–20 surement, particularly in the lower atmosphere where are viable out to 5 m. We can, however, take advantage refraction effects must be accounted for in extinctive of the slow motion of Earth relative to the background occultation observations. We will explore the basis for stars—it takes many days for Earth to progress in its these altitude ranges as we discuss our proof-of-concept orbit such that a given star is no longer available as an measurements with MSX/UVISI throughout this article. occultation source. Thus, a given star may be used many Owing to the low altitudes that must be reached times when it is observable, and the additional spectral before near-IR extinction is significant, an occultation coverage out to near-IR wavelengths is worth consider- technique combining extinctive and refractive mea- ation given the extended range of altitudes and species surements is both ideal and necessary: ideal because it enables. measuring the bulk atmosphere and the trace spe- cies simultaneously provides a complete picture of the We first give a brief historical perspective on stellar atmosphere, and necessary because refraction effects are occultation techniques to set the stage for a description intertwined with the extinction and must be separated. of the MSX/UVISI stellar occultation experiments. We Several hundred stars are bright enough and of an appro- then highlight the success of these proof-of-concept priate spectral class for far-UV and visible wavelength measurements and explore the future possibilities for the occultations. As wavelength increases, there are fewer combined stellar occultation technique as a means of stars of sufficient brightness or spectral class; approxi- probing Earth’s atmosphere. Extinctive Refractive Io(λ) I1(λ) I Star I2(λ) Star Io I3(λ) λ Refraction angle Relative motion Photometric of Earth or sensor End of atmosphere Central Star Flash 0 Flux 1 Figure 2. Geometric illustrations of the principles of various stellar occultation methods. In all three panels, the star is to the left and far enough away that the incoming rays can be considered parallel. The upper left panel shows the geometry of an extinctive occultation, in which the flux Io() from a star is progressively attenuated as the line of sight to the star moves deeper into the atmosphere. The ratio of the measured flux I at each point to the stellar flux is I/Io, known as the atmospheric (or stellar) transmission. Transmission values run from 0 (full extinction) to 1 (no extinction) and are a function of wavelength because the extinction by the atmosphere depends on the wavelength. The upper right panel shows the geometry of a refractive occultation, in which the angle between the observed ray and the unrefracted ray (the incoming stellar ray) is measured. The deeper in the atmosphere the line of sight penetrates, the larger the measured refraction angle will be.
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