Research Horizons

Solar-Cell Cover Glasses and Coatings

Several new types of lightweight, flexible, radiation-hardened coatings have been proposed to replace Ce-doped microsheet cover glass as encapsulation on solar cells. If successful, such coatings could drastically reduce costs and improve the capability of solar arrays. But before these materials can be deployed, a complete understanding of their performance in the space environment must be obtained. Aerospace researchers are investigating such coatings to provide critical data for evaluating end-of-life solar-array capabilities.

Simon Liu of the Aerospace Energy Technology Department and Michael Meshishnek of the Space Materials Laboratory are testing and modeling the effects of the space environment on solar-cell cover glasses and coatings. "Exposure of coating to the space environment often results in the loss of transmittance and reflectance due to defects generated from space radiation," said Liu. "The new coatings must protect the solar cells from space environmental elements such as space radiation, ultraviolet/visible radiation, atomic oxygen, thermal cycling, and high voltage discharge. The coatings also need to be highly transparent in the visible light spectrum and be radiation stable, allowing maximum power generation by the solar cells," Liu said.

"Aerospace is working to increase its ability to assess the technologies, to evaluate the state-of-the-art devices for specific satellite applications, and to advise the Air Force, national security space agencies, and NASA on investment and procurement strategy," Liu said. Aerospace has unique laboratory capabilities and experience to assess radiation degradation. For example, studies have been done in low-energy photon irradiation and combined proton-AM0 spectrum irradiation, low-energy electron, and ultraviolet irradiation sources.

"Aerospace also has excellent contacts with potential stakeholders in the new technologies—for example, developers, suppliers, and government programs," Liu said. Through involvement in the Small Business Innovation Research program, Aerospace has been participating in monitoring the development in industry of space photovoltaic-related technology.

During the first year of this three-year project, Aerospace has completed SRIM (Stopping and Range of Ions in Matter) modeling of proton penetration depth into polyhedral oligomeric silsesquioxane (POSS) coating for several simulated orbit environments. POSS embedded MA8000 (aromatic methacrylic) and PM1287 (vinylsilsesquioxane) host matrix with different dopants—10 percent Gd or 10 percent Tb—have been tested with low-energy protons at the Aerospace Low Energy Accelerator Facility. "Testing showed that the MA8000 POSS is not suitable as a coating for GaAs triple-junction solar cells," Liu said.

Computer simulation of atomic positions in a relaxed POSS (Si8O12H8) molecule.

Simulation of POSS molecule atomic positions after damage by space radiation.

Aerospace will continue to evaluate the POSS coating as the manufacturer makes improvements. The next generation POSS will be embedded in a DC93-500 host matrix for improved adhesion and radiation hardness. "Our efforts will focus on evaluating new POSS coatings for space environments, characterizing the defect generation mechanism in the coatings based on space environment effects, evaluating new coatings deposited with ion-assisted deposition and plasma-assisted deposition techniques for space radiation hardness, and establishing molecular dynamic simulation modeling for defect generating in coatings," Liu said.

Radiation assessment standards and databases are widely used to evaluate the end-of-life performance of solar arrays. At present, neither an assessment methodology nor a database exists for these advanced solar-cell coatings. The photovoltaic community is interested in developing lightweight and flexible coating to replace solar cell cover glass. Aerospace and the Air Force Research Laboratory have been cooperatively leading the study of the thin-film coating and thin-film photovoltaic-related technology for space applications.

Numerical Weather Prediction

By some estimates, watering the nation's lawns requires 200 gallons of water per person per day. What does that have to do with the weather?

Weather forecasts from numerical weather prediction models are required at increasingly high spatial and temporal resolution to support military operations and domestic emergencies. For example, high-resolution weather forecasts support satellite calibration and validation, launch load and plume predictions, and domestic emergency response planning, as well as routine air quality predictions.

The Aerospace Advanced Environmental Applications (AEA) group conducts research into numerical weather prediction using the complex Weather Research and Forecasting (WRF) model. WRF is a community model developed by the National Center for Atmospheric Research and is widely used in both civilian and DOD operational weather predictions. Aerospace runs the WRF model for Southern California every day (forecasts are posted at www.aerospaceweather.com). Aerospace specializes in using satellite data to improve the model forecasts. But recently, while conducting a series of forecasts over the Los Angeles Basin to help local air quality bureaus understand high ozone pollution episodes, Aerospace found that the benefits of satellite data can be masked by artifacts in the modeling system.

"We learned that the model tends to forecast warmer temperatures than observed," said Leslie Belsma, project engineer in AEA. "A source of cooling not included in the model system is anthropogenic moisture, or water from human processes. When water such as rain passes through dry air some of it evaporates, which requires energy. This energy is lost by the air and it represents a cooling process—the air gets cooler. The principle of evaporative coolers (swamp coolers) is the same. The two main human sources are domestic use—which consists of watering lawns, replenishing pools, and gardeners sweeping walks and driveways with water—and commercial crop irrigation, largely from the inland valleys' agricultural regions. Both of these sources act, in effect, like natural precipitation."

AEA commenced a study to find out if human water use does, in fact, influence the weather. The study, Belsma said, compares temperature forecasts made with the standard WRF model to forecasts made with a modified WRF model that incorporates anthropogenic water sources. Belsma's team, which includes Rich Coleman of the Sensors Signals and Electronics Subdivision and Jim Drake, Michael McAtee, and Al Fote of AEA, first gathered data to quantify how much water is being released into the atmosphere by human activities.

"Data for the 10 Southern California counties show that the combination of domestic use and commercial irrigation exceeds natural precipitation in five of them," Belsma said. "To incorporate anthropogenic moisture into the WRF model required distributing the water use in terms of the space and time scales needed by the model. Unfortunately, no single data set has all the required information. Aerospace developed a technique to infer the monthly use by mapping annual consumption amounts compiled by the USGS to month-by-month profiles for the 18 California evapotranspiration zones identified by the California Irrigation Management Information System program."

Daily anthropogenic equivalent precipitation for July in California. The scale is tenths of millimeters of equivalent daily precipitation. A sizable area of the state is affected by significant amounts of equivalent precipitation from watering lawns and crops, especially in summer when natural averages are less than 1.5 millimeters per month over most of the region.

The team then distributed the moisture spatially by mapping it to the satellite-derived land-use data embedded in WRF. Of the 24 land-use categories in WRF, Belsma said, the team distributed the moisture over the four allocated to irrigated croplands and one designated as urban land. "For the month of July, data show that a sizable area of the state is affected by significant amounts of 'equivalent precipitation' from watering our lawns and crops—especially in summer, when natural precipitation averages less than 1.5 millimeters per month over most of the region."

Next, the researchers had to modify the WRF model so that it could include the water from human activities in calculations of how surface properties affect the heating or cooling of the atmosphere. They added the moisture data to a registry from which WRF computed the added water at each model time step. This was combined with any rain at the surface within the WRF land surface submodel.

"We used our normal daily runs over Southern California, choosing 30 days between July and August of 2007 as the 'control run,' i.e., the model forecast generated without the domestic and irrigation water. We then ran the modified WRF for the same dates. We compared the forecasts from both model runs to surface observations of temperature throughout the entire region. When we computed the temperature bias—that is, whether the forecast temperature tended to be consistently warmer or colder than what was observed—we found that the temperature bias varied dramatically with the time of day, but was very similar for the modified and control model runs," Belsma said.

The greatest difference between the control and the irrigated run was at the 19th forecast hour (noon local time), when the bias of the modified case was slightly (0.13 K) cooler. To see if the differences might stand out more if the team just compared model forecasts to observations located in the areas in which they inserted the anthropogenic moisture, they restricted the calculation of the bias in temperature to just these areas.

"The biases showed the same strong diurnal variation and again were generally close to each other, with the bias a bit cooler (0.20 K) compared to the control run. We next looked at the effect at a single station, the Bakersfield airfield, located at the southeast corner of the extensively irrigated San Joaquin Valley. Averaged over all days and forecast hours the temperatures forecast at Bakersfield were nearly 1 degree cooler than the forecasts made without taking into account this water source," Belsma said.

Belsma said the team's study demonstrated the impact of human water consumption on the weather: "We already know that water consumption is a major environmental issue worldwide. Now Aerospace has demonstrated that all that water going to quench the thirst of our lawns and agricultural regions in this desert climate of Southern California affects the weather as well."

Effect of Solar Energetic Particles on the Space Environment

Energetic particles that are accelerated at solar flares and travel toward Earth at near relativistic speeds pose a serious threat to Earth-orbiting spacecraft. Often, spacecraft passing through penetrating radiation from these energetic particles see a complex structure that includes large latitudinal and longitudinal variations in particle fluence. Thus a strong likelihood exists that the actual environment may be significantly different than current models indicate. These variations have only begun to be explained by numerical trajectory calculations. Inclusion of the interplanetary magnetic field, which guides these particles toward Earth, is of paramount importance in understanding how they penetrate Earth-orbiting spacecraft.

A team of Aerospace scientists—Tamitha Skov and Joseph Mazur of the Aerospace Space Sciences Department and J. Bernard Blake of the Space Science Applications Laboratory—has been using multispacecraft measurements of upstream magnetic fields to further develop the particle model that traces numerical trajectories of energetic particles through the complex structure of the solar wind. "The ultimate goal is to provide realistic particle trajectories and their incidence at Earth," said Skov. "This will allow a better prediction of the variation of penetrating radiation seen deep within the magnetosphere and at high latitudes, where penetrating radiation is most prominent. These analyses will help generate better specifications of energetic protons, electrons, and ions that have not been modeled well in the past."

Accurate predictions of the distribution of charged particles in the inner magnetosphere are needed for spacecraft requirements and simulations of space environmental effects on materials. "It is clear from current missions that the inner magnetosphere is highly dynamic, and the extremes in energetic particle fluence are yet unknown," Skov said. "Within the past decade, spacecraft technology has advanced far beyond our current specifications. At the same time, national security space interest in more hazardous orbits has far outpaced the development of new models in these regions." Existing trapped-radiation models treat solar-wind and solar-cycle variability extremely crudely and are only beginning to provide probability distributions as part of their output.

The team has extended the energetic particle anisotropy database compiled from the NASA Advanced Composition Explorer (ACE) and Polar satellites from FY06 by including data from the NASA Solar Anomalous Component Magnetospheric Particles Explorer (SAMPEX) and NOAA's Geostationary Operational Environmental Satellites (GOES), as well as new data from Polar. ACE samples low-energy particles of solar origin as well as higher-energy galactic particles. SAMPEX is designed to detect solar energetic particles, precipitating energetic electrons, anomalous cosmic rays, and galactic cosmic rays. Polar measures energetic particles and energy input to the Earth's polar regions. GOES monitors Earth's weather and the near-Earth space environment.

Diagram of the 3-D orientation of a solar wind transient structure as it passes the orbit of Earth. The numerical trajectories of solar energetic particles streaming through the center of the structure having energies of 20 MeV, 45 MeV, and 110 MeV are superposed. Note how the particle paths bend sharply at the same location, which corresponds to the magnetic boundary between the transient structure and the associated interplanetary shock (not shown).

Skov said that the team has made several significant accomplishments: It has completed a numerical trajectory-tracing model through the L1 libration point to the magnetopause for quiet and disturbed solar wind periods. The L1 libration point is approximately one one-hundredth of the way from Earth to the sun (e.g., about 1.5 million kilometers, 1 million miles, or 240 Earth radii in front of Earth), where the gravitational and orbital forces of Earth and the sun are balanced, allowing a spacecraft to remain in a relatively stationary location relative to Earth as it orbits the sun. Interplanetary observations upstream of Earth—specifically, ACE and Wind spacecraft data at the L1 libration point—have been imported into the model. A preliminary 3-D field-line mapping of the spatial domain within the model has been created to determine field gradients at the boundaries of the inverted 3-D magnetic structures.

The team also determined that highest energy particles can remain trapped in magnetic mirrors convecting with solar wind. These oscillating particles are observed at the Polar and SAMPEX spacecraft. New evidence shows that geosynchronous Earth orbits have unusually isotropic particle fluences during solar particle events; electron precursors of these events, as detected in SAMPEX and high Earth orbit spacecraft data, may possibly aid in prediction.

The team plans next to enhance the numerical trajectory-tracing model by incorporating multispacecraft observations of the interplanetary field upstream of Earth, including the twin STEREO spacecraft. Adding the solar wind interplanetary magnetic field variability as a real-time parameter in the model will enable the examination of modulation effects on the energetic particle fluence anisotropy and the way in which the anisotropy varies deep within the magnetosphere compared to high latitudes. The team will also begin to analyze extended data sets for solar-cycle effects in penetrating radiation variability.

Plans are also to improve the 3-D field-line mapping of the solar wind with the transient model field inversions to better understand the field gradients that occur when multiple observed and modeled field data sets are coupled. Analysis will continue of the energetic particle fluence at the solar wind monitor position ahead of Earth for comparison with predicted numerical trajectories and the variability seen in Polar pitch-angle distributions. SAMPEX measurements will also continue regarding how the solar wind interplanetary magnetic field drives radiation deep in the magnetosphere during periods of quiet and high solar wind variability.

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