Lasers Probe Atmosphere for Aerosol Characterization
First published Summer 2011, Crosslink® magazine.
Aerosols are the fine particulates suspended in the air that produce hazy conditions. These small particles play a critical role in climate and weather, and directly affect how much solar energy is retained or reflected by Earth’s atmosphere. Their primary influence is through scattering of solar radiation, but they also can absorb a significant amount of energy, depending on their composition. Aerosols also play a secondary role as the nuclei for condensation of water and other atmospheric species to form fog and clouds.
While the critical role of aerosols in climate and weather is acknowledged, their exact contribution is poorly understood, said Pavel Ionov of Aerospace’s Photonics Technology Department. In fact, he explained, aerosols represent one of the largest sources of uncertainty in climate models because they are incredibly complex and diverse, as are the mechanisms of their creation, transformation, and removal from the atmosphere. In addition, their relatively short lifetimes of one to two weeks lead to incomplete mixing and very complex spatial (especially vertical) distributions.
“Aerosol effects are not limited to weather and climate,” Ionov said. “For instance, they play an important role in atmospheric chemistry and public health. Also of interest to Aerospace’s primary customers is the role aerosols play in degrading space-based imaging—especially hyper-spectral—as well as affecting laser propagation through the atmosphere.”
The Photonics Technology Department has been developing laser-based remote sensing of atmospheric aerosols. This technique, known as a lidar, probes the vertical distribution of aerosols in the atmosphere. The system sends a short pulse of laser light vertically into the atmosphere, and some of the laser light scatters back off of the aerosols and air molecules. The time of arrival of the scattered light provides distance information, and the intensity of the backscattered signal reveals how much aerosol is present.
The primary focus of the Aerospace aerosol lidar program has been calibration and validation of satellite sensors. Ionov, together with Steven Beck of the Electronics and Photonics Laboratory, Leslie Belsma of Environmental Satellite Systems, and Christopher Woods of Radar and Signal Systems, have been working on a research project to improve aerosol optical thickness measurements from space using lidar ground truth data. “Because of our poor understanding of aerosol mechanisms, direct monitoring from space is the only viable way to assess their global weather and climate effects,” explained Beck, who has been working with lidar since its inception.
The MODIS (Moderate Resolution Imaging Spectroradiometer) sensor onboard NASA’s Aqua and Terra satellites provides optical thickness data, and a new VIIRS (Visible/Infrared Imager Radiometer Suite) instrument is planned for future National Oceanic and Atmospheric Administration Joint Polar Satellite System and Air Force Defense Weather Satellite System missions. “Despite the great need for global coverage, passive remote sensing of aerosols from space is fraught with uncertainties,” Ionov said. “An unknown Earth surface albedo presents the greatest challenge. Thus, ground-based active and passive remote sensing is critical to orbital sensor calibration.”
All Earth-sensing satellite instruments require ground-truth validation once on orbit—satellite data must be compared with known and verifiable data taken at the same time and location by ground or aircraft based sensors. The primary goal of the Aerospace aerosol program is to develop reliable remote sensing techniques that provide such calibration standard while providing as much information about the aerosols as possible. One approach is to use multiple instruments for greater accuracy and the complementary information that they can provide. The Aerospace project also operates a collocated sun photometer. This instrument derives height-integrated aerosol parameters from solar and sky radiance measurements. This combination of active and passive aerosol instruments creates a more reliable and more comprehensive data set than any one of the instruments can provide. The combined data is continuously collected, checked for consistency, and archived. It is then further compared with the aerosol data products of space-based sensors such as MODIS.
In addition to the lidar and sun photometer data, other meteorological data is combined into a comprehensive database. This database serves as a unique resource for exploring the relationships between aerosols and local meteorological conditions. The research team is hoping that this will provide insight into aerosol production and transport mechanisms. This knowledge will also improve accuracy of space-based remote sensing of aerosols. Because of the complexity and variety of aerosols and because satellite measurements of them are indirect, analysis of satellite data is complex and relies on assumptions about likely aerosol compositions.
Improving the scientific understanding of aerosol mechanisms will improve these assumptions and the algorithm accuracy of space-based sensors such as MODIS and VIIRS.
A unique feature of lidar is its remarkable spatial and temporal resolution. The lidar data shown in the accompanying graphic reveals complex local atmospheric dynamics. “It is the study of the mechanisms underlying the aerosol dynamics that is of interest to the research community,” said Beck. The research team is hoping that their measurements will find application in areas that go beyond space sensor calibration. For example, aerosols serve as a convenient marker to visualize atmospheric boundary layer dynamics, which, in turn, is critical for accurate weather modeling and in study of pollution transport.
Building Plasma Specifications for Highly Elliptical Orbits
Space plasma contributes to two distinct spacecraft hazards: surface charging and surface dose. All satellites, in all orbits, have surfaces that charge in response to the space plasma environment. Because differential charging carries an associated discharge risk, satellites must mitigate surface charging. In addition, sensitive satellite surfaces can degrade as a result of the dose accumulated from that same space plasma environment. The environment responsible for surface charging and dose is only well observed near geosynchronous Earth orbit (GEO).
A team of Aerospace scientists, led by Timothy Guild of the Space Science Department, is working to broaden the knowledge of this space plasma environment beyond GEO by using unique instruments in highly elliptical orbits (HEO). The team will develop two plasma specifications specifically tailored to HEO. One will characterize space plasmas that contribute to surface dose, while the other will determine the plasma environment most conducive to surface charging. “These specifications will feed directly into ongoing spacecraft development efforts and aid in evaluating on-orbit anomalies related to surface charging,” Guild said.
The electrostatic potential of spacecraft surfaces is a complex function of the net current to those surfaces from the space environment. Low-energy ions and electrons impact the surface and impart their charge, or, depending on the surface and incident species, eject one or more secondary electrons. Ultraviolet photons on the sunlit side remove charge through photoionization. Any surface that charges to a large potential relative to neighboring surfaces poses a potential discharge risk, and requires mitigation.
“One shortcoming of existing surface charging specifications is that they were largely derived from measurements at GEO,” Guild said. “The process of surface charging can be highly localized, even within a few hours of local time at GEO. Previous HEO observations of charging intervals show a strong radial, local time, and geomagnetic activity dependence to the charging likelihood.”
Guild and other members of the team—Joseph Fennell, James Roeder, James Clemmons, and Margaret Chen, also from the Space Science Department—are using these plasma observations in HEO, as well as observations from the Aerospace-developed surface-charging monitors on the NASA TWINS (Two Wide-angle Imaging Neutral-atom Spectrometers) mission to investigate charging intervals in HEO. The charged particle motion in space allows these HEO observations to be mapped along magnetic field lines to other orbits, contributing to charging specifications for orbits from medium Earth orbit out beyond GEO.
The impinging ions and electrons deposit all their energy in the first few mils (1 mil = 0.001 inch) of the spacecraft surface, causing intense radiation damage to thin films and coatings. The surface radiation dose caused by the low-energy plasma dominates for thicknesses below about 1 mil.
“Existing satellite specifications for surface dose are also largely GEO-centric,” Guild said. “In our project, we are developing a surface dose specification for vehicles that traverse HEO magnetic field lines, sometimes flying through a very different plasma environment.” Guild noted that previous Aerospace research showed order-of-magnitude differences between the average omnidirectional hydrogen flux between GEO and GPS orbits. “By combining these three specialized HEO plasma datasets, we will drastically improve our knowledge of the environment in time and space, leading to a more robust and more widely applicable plasma specification for surface dose,” he said.
Current understanding of surface dose and charging, as well as the state-of-the-art specifications of these hazards, have been contributed by The Aerospace Corporation, NASA, and Los Alamos National Laboratory, among other institutions. Aerospace personnel are widely recognized in the fields of spacecraft surface dose and surface charging, and have contributed many of the plasma and surface charging specifications used for spacecraft design.
“Aerospace has designed, built, and operates three plasma analyzers in HEO uniquely suited to providing plasma and surface charging specifications,” Guild said. “Aerospace personnel have the expertise to appropriately interpret these observations and their differences with other empirical models. After developing the plasma and surface charging specifications, Aerospace is also well positioned to include these results into the next-generation radiation specification models and effectively communicate the results to relevant customers via our close involvement with many national space programs,” Guild said.
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