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NASA NTRS1977

Inversion of passive microwave remote sensing data from satellites

Global passive microwave observations from earth-orbiting satellites have mapped humidity and liquid water over ocean, temperature profiles, ice and snow, and other geophysical parameters. In most applications, the inversion problem is adequately approximated as linear with jointly Gaussian statistics, and, thus, a linear retrieval performs well. In some cases, the problem is typically factored into a decision process followed by appropriate linear or quasilinear processes. Certain problems, however, require more powerful nonlinear or nonstationary procedures, such as Kalman filtering.

Staelin, D. H.↗

NASA NTRS1978

The association of nonthermal electrons with non-flaring coronal transients

A close temporal and spatial association has been found between erupting filaments/coronal transients and radio noise storm continua. The three transients studied occurred away from active regions and are members of a class not usually accompanied by chromospheric emission. The data analyzed were from the S-054 soft X-ray telescope on Skylab and from one- and two-dimensional interferometers at meter and decameter wavelengths at Nancay and Clark Lake observatories. Calculations confirmed that observed microwave radiation from the transients is due to thermal bremsstrahlung. The results are consistent with an interpretation of heating of an increased amount of coronal plasma by nonthermal, 10-100 keV electrons. Three possibilities for the source of the material are described: (1) the filament material; (2) evaporation from the chromosphere or transition region; and (3) via a long-lived reconnection process.

Webb, D. F.↗

NASA NTRS1979

A review of short-term flare forecasting activities at Toyokawa

Short-term forecasting of solar activity conducted at Toyokawa is briefly reviewed. The forecasts are based on microwave observations of the slowly varying component of solar radiation associated with active regions. It was found that for proton flares which take place in intense active regions, it is possible to predict their occurrence with certain accuracy, but for a certain class of proton flares which occur in a small active region or which are associated with an active region, which evolves very rapidly, or for whose which occur behind the limb, it is almost impossible to forecast their occurrence.

Enome, S.↗

NASA NTRS1979

The mission of Seasat-A

The experimental oceanographic satellite Seasat-A produced microwave observations of the world's oceans for some three months in the summer of 1978. Study of a small set of the data collected at a time and in the area of intensive conventional surface observations have shown that the technology represented by this satellite can provide useful information on waves, winds and sea surface temperature on a global scale. Further analysis is required to quantitatively assess the potential contribution of satellite height and orbit determination to the study ocean dynamics, but early results are quite encouraging.

Dunne, J. A.↗

NASA NTRS1981

A simulation study of the recession coefficient for antecedent precipitation index

The antecedent precipitation index (API) is a useful indicator of soil moisture conditions for watershed runoff calculations and recent attempts to correlate this index with spaceborne microwave observations have been fairly successful. It is shown that the prognostic equation for soil moisture used in some of the atmospheric general circulation models together with Thornthwaite-Mather parameterization of actual evapotranspiration leads to API equations. The recession coefficient for API is found to depend on climatic factors through potential evapotranspiration and on soil texture through the field capacity and the permanent wilting point. Climatologial data for Wisconsin together with a recently developed model for global isolation are used to simulate the annual trend of the recession coefficient. Good quantitative agreement is shown with the observed trend at Fennimore and Colby watersheds in Wisconsin. It is suggested that API could be a unifying vocabulary for watershed and atmospheric general circulation modelars.

Choudhury, B. J.↗

NASA NTRS1981

High-resolution passive microwave imaging of the surface of the Earth

The physics of passive microwave observations of the Earth and the system requirements for high-resolution imaging within this spectral band are summarized. High resolution is achieved in a straightforward manner by increasing the size of the primary antenna. However, with a single receiver, it is shown that the combination of high resolution and crosstrack scanning cannot produce images which have valuable geophysical content. The concept of a multiple receiver array located in the focal plane is presented as the only practical solution to the dilemma. Exploring this concept, system requirements are generated which, for the first order, appear to offer solutions to the problem.

Swift, C. T.↗

NASA NTRS1983

Speckle interferometry of IRC +10216 in the fundamental vibration-rotation lines of CO

The largest fraction of the matter returned by stars to the interstellar medium is probably provided by red giants. The carbon star IRC +10216 is an example of an evolved giant with a large mass loss rate. One plausible mechanism for the acceleration of the gas in stars like IRC +10216 is radiation pressure on dust grains which then collide with and transfer their momentum to the gas. However, at the present time neither infrared nor microwave observations provide a clear picture of the distribution of matter near cool red giant stars. There exists one method which may be used to obtain more information about the distribution of matter very close to the star. This method involves the measurement of the spatial extent of near-infrared lines by employing a combination of very high spatial and high spectral resolution. The present investigation is concerned with an application of this method. Speckle interferometry is used to measure the radial distribution of CO molecules on angular scales of 1 sec near IRC +10216.

Dyck, H. M.↗

NASA NTRS1984

Inversion Algorithms for Water Vapor Radiometers Operating at 20.7 and 31.4 Ghz

Eight water vapor radiometers (WVRs) were constructed as research and development tools to support the Advanced System Programs in the Deep Space Network and the Crustal Dynamics Project. These instruments are intended to operate at the stations of the Deep Space Network (DSN), various radio observatories, and obile facilities that participate in very long baseline interferometric (VLBI) experiments. It is expected that the WVRs will operate in a wide range of meteorological conditions. Several algorithms are discussed that are used to estimate the line-of-sight path delay due to water vapor and columnar liquid water rom the observed microwave brightness temperatures provided by the WVRs. In particular, systematic effects due to site and seasonal variations are examined. The accuracy of the estimation as indicated by a simulation calculation is approximately 0.3 cm for a noiseless WVR in clear and moderately cloudy weather. With a realistic noise model of WVR behavior, the inversion accuracy is approximately 0.6 cm.

Resch, G. M.↗

NASA NTRS1982

CONSERVB: A numerical method to compute soil water content and temperature profiles under a bare surface

A comprehensive, yet fairly simple model of water disposition in a bare soil profile under the sequential impact of rain storms and other atmospheric influences, as they occur from hour to hour is presented. This model is intended mostly to support field studies of soil moisture dynamics by our current team, to serve as a background for the microwave measurements, and, eventually, to serve as a point of departure for soil moisture predictions for estimates based in part upon airborne measurements. The main distinction of the current model is that it accounts not only for the moisture flow in the soil-atmosphere system, but also for the energy flow and, hence, calculates system temperatures. Also, the model is of a dynamic nature, capable of supporting any required degree of resolution in time and space. Much critical testing of the sample is needed before the complexities of the hydrology of a vegetated surface can be related meaningfully to microwave observations.

Vanbavel, C. H. M.↗

NASA NTRS1983

The NASA SETI Program - Objectives of the R&D activity

The current activity in NASA's SETI Program is a research and development program jointly carried out by the NASA Ames Research Center and the Jet Propulsion Laboratory. The purpose of this R&D phase is to develop all prerequisites for a well defined microwave observing program. The specific objectives include the development of a prototype search system which will ultimately be capable of processing data rates up to 10 gigabytes/second. Specialized signal detection algorithms and automated search procedures will be developed and tested with the prototype hardware. A series of field tests will be carried out with the NASA Deep Space Network facilities at Goldstone, CA and with the 305-m radio telescope at the Arecibo Ionospheric Observatory in Puerto Rico.

Klein, M. J.↗

NASA NTRS1984

RS CV sub n binary systems

An attempt is made to place in context the vast amount of data obtained as a result of X-ray, ultraviolet, optical, and microwave observations of RS CVn and similar spectroscopic binary systems. Emphasis is on the RS CVn systems and their long period analogs. The following questions are considered: (1) are the original defining characteristics still valid and still adequate? (2) what is the evidence for discrete active regions? (3) have any meaningful physical properties for the atmospheres of RS CVn systems been derived? (4) what do the flare observations tell about magnetic fields in RS CVn systems? (5) is there evidence for systematic trends in RS CVn systems with spectral type?

Linsky, J. L.↗

NASA NTRS1985

SETI - The microwave search problem and the NASA sky survey approach

NASA's microwave observing program for SETI is presented. This strategy is composed of a high sensitivity, narrow frequency coverage, Target Search and a low sensitivity, broad frequency coverage, Sky Survey. The complementary nature of this dual mode search strategy is discussed. An overview is given of ongoing work in the development of the search strategy for the Sky Survey.

Klein, M. J.↗

NASA NTRS1986

The microwave spectrum of solar millisecond spikes

The microwave radiation from solar flares sometimes shows short and intensive spikes which are superimposed on the burst continuum. In order to determine the upper frequency limit of their occurrence and the circular polarization, a statistical analysis was performed on digital microwave observations from 3.2 to 92.5 GHz. Additionally, fine structures were investigated with a fast 32-channel spectrometer at 3.47 GHz. It was found that about 10 percent of the bursts show fine structures at 3.2 and 5.2 GHz, whereas none occurred above 8.4 GHz. Most of the observed spikes were very short and their bandwidth varied from below 0.5 MHz to more than 200 MHz. Simultaneous observations at two further frequencies showed no coincident spikes at the second and third harmonic. The observations can be explained by the theory of electron cyclotron masering if the observed bandwidths are determined by magnetic field inhomogeneities or if the rise times are independent of the source diameters. The latter would imply source sizes between 50 and 100 km.

Staehli, M.↗

NASA NTRS1986

Particle propagation effects on wave growth in a solar flux tube

The evolution of a distribution of electrons is followed after they are injected impulsively at the top of a coronal magnetic loop, with the objective of studying the plasma instabilities which result. At early times the downgoing electrons have beamlike distributions and amplify electrostatic waves via the Cerenkov resonance; the anomalous Doppler resonance is found to be less important. Slightly later, while the electrons are still predominantly downgoing, they are unstable to cyclotron maser generation of z-mode waves with omega(p) much less than Omega, or to second harmonic x-mode waves. The energetics of these instabilities, including saturation effects and heating of the ambient plasma, are discussed. It is suggested that coalescence of two z-mode waves generated by cyclotron maser emission of the downgoing electrons may produce the observed microwave spike bursts.

White, S. M.↗

NASA NTRS1986

Evaluation of the microwave spectrum of Venus in the 1.2-22 centimeter wavelength range based on laboratory measurements of constituent gas opacities

Laboratory measurements of the microwave (1.2-22.3 cm) spectrum properties of Venus' gaseous atmospheric constituents were performed using an apparatus for simulating conditions of the middle atmosphere of Venus (gaseous H2SO4 + CO2 at 1 to 6 atm). The results have shown that at wavelengths longer than 1.8 cm, gaseous H2SO4 and CO2 are the predominant microwave absorbers, while at wavelengths from 1.2 to 1.8 cm, SO2 and CO2 are the predominant absorbers. These results were used to develop a model for the microwave emission spectrum of Venus, which correlated well with microwave observations of this planet.

Steffes, Paul G.↗

NASA NTRS1986

Solar burst precursors and energy build-up at microwave wavelengths

We summarize high-resolution microwave observations (VLA) of heating and magnetic triggering in coronal loops. Magnetic changes that precede solar eruptions on time scales of tens of minutes involve primarily emerging coronal loops and the interaction of two or more loops. Thermal cyclotron lines have been detected in coronal loops, suggesting the presence of hot current sheets that enhance emission from relatively thin layers of enhanced temperature and constant magnetic field. These current sheets may play a role in the excitation of solar bursts. A filament-associated source with a high brightness temperature and steep radiation spectrum occurs above a region of apparently weak photospheric field. This source might be attributed to currents that enhance coronal magnetic fields. Compact (phi=5 sec) transient sources with lifetimes of 30 to 60 minutes have also been detected in regions of apparently weak photospheric field. We conclude by comparing VLA observations of coronal loops with simultaneous SMM-XRP observations.

Lang, Kenneth R.↗

NASA NTRS1987

Intelligent signal analysis and recognition

Progress in the research and development of self-organizing database system that can support the identification and characterization of signals in an RF environment is described. As the radio frequency spectrum becomes more crowded, there are a number of situations that require a characterization of the RF environment. This database system is designed to be practical in applications where communications and other instruments encounter a time varying and complex RF environment. The primary application of this system is the guidance and control of NASA's SETI Microwave Observing Project. Other possible applications include selection of telemety bands for communication with spacecraft, and the scheduling of antenna for radio astronomy are two examples where characterization of the RF environment is required. In these applications, the RF environment is constantly changing, and even experienced operators cannot quickly identify the multitude of signals that can be encountered. Some of these signals are repetitive, others appear to occur sporadically.

Levinson, Robert↗

NASA NTRS1987

The role of aqueous chemistry in determining the composition and cloud structure of the upper troposphere on Uranus

Aqueous chemistry on Uranus affects the atmospheric abundances of NH3 and H2S below the methane cloud base. Here a complete thermochemical equilibrium model for the H2O-NH3-H2S system is presented. Inclusion of H2S increases the aqueous removal of NH3 to 20-30 percent, but aqueous chemistry alone cannot account for the depletion of NH3 in the 150-200-K region of the atmosphere required to fit microwave observations. Formation of NH4SH clouds can account for the observed depletion provided the H2S/NH3 ratio is enhanced by a factor of 4 relative to solar. Perturbations to the chemical balance between N and S, for example by the general circulation on Uranus, would then produce regions with either NH3 or H2S aloft.

Carlson, Barbara E.↗