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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Dust transport in the Martian atmosphere

Dust in suspension within the martian atmosphere is an important driver of the atmospheric thermal and, ultimately, dynamical states. By virtue of its presence in the atmosphere, this suspended dust is susceptible to transport by the winds and thus the location from which dust is lifted from the surface is not necessarily the location at which it will return to the surface. Such surface dust lifting and subsequent transport, when accumulated over daily, seasonal and annual time intervals define the martian dust cycle. The atmospheric transport element (and equally surface lifting and redeposition) of this cycle is strongly dependent upon season and the quantity of dust in suspension. The seasonal dependence arises due to the dominance of particular components of the atmospheric circulation at particular locations (latitudes) at various times through the year. The dependence on dust abundance is due to the amount of dust available for transport, but it is also due to the influence that the dust has on the intensity of the circulation. As dust abundance and therefore its radiative influence increases, some components of the circulation intensity (Hadley circulation, thermal tides), while some components may decline in intensity (baroclinic waves, condensation flow). For these reasons, our ability to understand the martian dust cycle is dependent on our ability to define the contributions that various circulation components, under varying seasonal and dust-radiative forcings, play in the transport of dust around the planet.

Murphy, James R.↗
NASA NTRS1991

Anisotropic inverse Compton emission

The angular distribution of the power and spectrum of Compton-scattered radiation in the case of an anisotropic distribution of soft photons is considered. Results obtained show that both the intensity and spectrum of the scattered radiation are a function of the Lorentz factor of the electrons, the geometry of the soft X-ray source and the viewing angle. The intensity of the hard X-rays scattered back into the bowl can be significantly greater (by a factor of up to 5.5) than that scattered toward the observer for an isotropic distribution of relativistic electrons at the center of a hemispherical bowl of soft X-ray emission. This factor is increased as the solid angle of the soft X-ray source subtended at the location of the electrons is reduced. The results obtained here have inmplications for models invoking a further 'reflection' of the resultant hard X-rays from the surface of a disk.

Ghisellini, G.↗
NASA NTRS2005

Cassini Radio and Plasma Wave Observations at Saturn

Results are presented from the Cassini radio and plasma wave instrument during the approach and first few orbits around Saturn. During the approach the intensity modulation of Saturn Kilometric Radiation (SKR) showed that the radio rotation period of Saturn has increased to 10 hr 45 min plus or minus 36 sec, about 6 min longer than measured by Voyager in 1980-81. Also, many intense impulsive radio signals called Saturn Electrostatic Discharges (SEDs) were detected from saturnian lightning, starting as far as 1.08 AU from Saturn, much farther than terrestrial lightning can be detected from Earth. Some of the SED episodes have been linked to cloud systems observed in Saturn s atmosphere by the Cassini imaging system. Within the magnetosphere plasma wave emissions have been used to construct an electron density profile through the inner region of the magnetosphere. With decreasing radial distance the electron density increases gradually to a peak of about 100 per cubic centimeter near the outer edge of the A ring, and then drops precipitously to values as low as .03 per cubic centimeter over the rings. Numerous nearly monochromatic whistler-mode emissions were observed as the spacecraft passed over the rings that are believed to be produced by meteoroid impacts on the rings. Whistlermode emissions, similar to terrestrial auroral hiss were also observed over the rings, indicating that an electrodynamic interaction, similar to auroral particle acceleration, may be occurring in or near the rings. During the Titan flybys Langmuir probe and plasma wave measurements provided observations of the density and temperature in Titan's ionosphere.

Gurnett, D. A.↗
NASA NTRS1998

Inverse Compton Scattering in Mildly Relativistic Plasma

We investigated the effect of inverse Compton scattering in mildly relativistic static and moving plasmas with low optical depth using Monte Carlo simulations, and calculated the Sunyaev-Zel'dovich effect in the cosmic background radiation. Our semi-analytic method is based on a separation of photon diffusion in frequency and real space. We use Monte Carlo simulation to derive the intensity and frequency of the scattered photons for a monochromatic incoming radiation. The outgoing spectrum is determined by integrating over the spectrum of the incoming radiation using the intensity to determine the correct weight. This method makes it possible to study the emerging radiation as a function of frequency and direction. As a first application we have studied the effects of finite optical depth and gas infall on the Sunyaev-Zel'dovich effect (not possible with the extended Kompaneets equation) and discuss the parameter range in which the Boltzmann equation and its expansions can be used. For high temperature clusters (k(sub B)T(sub e) greater than or approximately equal to 15 keV) relativistic corrections based on a fifth order expansion of the extended Kompaneets equation seriously underestimate the Sunyaev-Zel'dovich effect at high frequencies. The contribution from plasma infall is less important for reasonable velocities. We give a convenient analytical expression for the dependence of the cross-over frequency on temperature, optical depth, and gas infall speed. Optical depth effects are often more important than relativistic corrections, and should be taken into account for high-precision work, but are smaller than the typical kinematic effect from cluster radial velocities.

Molnar, S. M.↗
NASA NTRS1976

Inhomogeneities in the early universe

The paper investigates certain nonlinear processes that are viable candidates for the mechanisms which produced large-scale inhomogeneities in the early Universe. Several nonlinear Lagrangians are presented for matter, the Korteweg-de Vries equation is analyzed, and the existence of solitons among its solutions is noted. A model based on the possibility of generating a cascade of solitons from an initial perturbation is proposed, and it is shown how large-scale inhomogeneities can be generated when an initial soliton fragments into many others through the nonlinear action of the terms in the Korteweg-de Vries equation. A second model is examined which is based on the interaction of matter with a strong radiation field (an almost monochromatic photon gas) and which involves changes in the refractive index of the vacuum. It is found that matter and radiation will not mix if the radiation field has a nonuniform intensity and that the matter will separate into dense portions or 'cosmological protogalaxies'. The evolution of these portions of matter is studied, and it is found that conditions would be appropriate for the interface between them and the surrounding radiation field to become unstable, giving rise to a turbulent layer.

Canuto, V.↗
NASA NTRS1986

Probing the inner regions of the coma by UV observations of CS in comets

New vibrational bands of CS have been identified in several comets. The relative intensities of these bands are in agreement with a simple model, based upon most of the CS radicals being in the lowest vibrational level. The present observational accuracy is not sufficient to decide between this simple model and an earlier radiative equilibrium model. The observed intensity ratios appear to be similar in different comets, which means that they cannot depend strongly upon the total gas production rates.

Butterworth, P. S.↗
NASA NTRS1985

The detailed intensity distribution of the AKR emission cone

The detailed intensity distribution of auroral kilometric radiation (AKR) at frequencies near the spectral peak of 200 kHz and below, over the entire northern hemisphere at large radial distances, is presented based on 3.5 years of IMP 6 data and four years of Hawkeye data. The time-averaged intensity of the AKR emission cone at frequencies from 178 to 100 kHz is found to be nearly uniform. Evidence contradicts a 'hollow cone' structure. The solid angles for the AKR emission cones studied are in good agreement with previously published results, and there is good agreement with previously determined AKR source location. The time-averaged picture of the AKR emission pattern at 178 kHz is representative of the instantaneous AKR emission. Present theories for the generation and propagation of AKR appear insufficient to explain the observed illumination pattern.

Green, J. L.↗
NASA NTRS2003

Estimates of the Spectral Aerosol Single Sea Scattering Albedo and Aerosol Radiative Effects during SAFARI 2000

Using measurements of the spectral solar radiative flux and optical depth for 2 days (24 August and 6 September 2000) during the SAFARI 2000 intensive field experiment and a detailed radiative transfer model, we estimate the spectral single scattering albedo of the aerosol layer. The single scattering albedo is similar on the 2 days even though the optical depth for the aerosol layer was quite different. The aerosol single scattering albedo was between 0.85 and 0.90 at 350 nm, decreasing to 0.6 in the near infrared. The magnitude and decrease with wavelength of the single scattering albedo are consistent with the absorption properties of small black carbon particles. We estimate the uncertainty in the single scattering albedo due to the uncertainty in the measured fractional absorption and optical depths. The uncertainty in the single scattering albedo is significantly less on the high-optical-depth day (6 September) than on the low-optical-depth day (24 August). On the high-optical-depth day, the uncertainty in the single scattering albedo is 0.02 in the midvisible whereas on the low-optical-depth day the uncertainty is 0.08 in the midvisible. On both days, the uncertainty becomes larger in the near infrared. We compute the radiative effect of the aerosol by comparing calculations with and without the aerosol. The effect at the top of the atmosphere (TOA) is to cool the atmosphere by 13 W/sq m on 24 August and 17 W/sq m on 6 September. The effect on the downward flux at the surface is a reduction of 57 W/sq m on 24 August and 200 W/sq m on 6 September. The aerosol effect on the downward flux at the surface is in good agreement with the results reported from the Indian Ocean Experiment (INDOEX).

Bergstrom, Robert W.↗
NASA NTRS1980

Some experience with arc-heater simulation of outer planet entry radiation

An electric arc heater was operated at 800 amperes and 100,000 pa (1 atm) with hydrogen, helium, and two mixtures of hydrogen and helium. A VUV-scanning monochromator was used to record the spectra from an end view while a second spectrometer was used to determine the plasma temperature using hydrogen continuum radiation at 562 nm. Except for pure helium, the plasma temperature was found to be too low to produce significant helium radiation, and the measured spectra were primarily the hydrogen spectra with the highest intensity in the pure hydrogen case. A radiation computer code was used to compute the spectra for comparison to the measurements and to extend the study to simulation of outer planet entry radiation. Conductive cooling prevented ablation of phenolic carbon material samples mounted inside the arc heater during a cursory attempt to produce radiation absorption by ablation gases.

Wells, W. L.↗
NASA NTRS1990

ERBE and AVHRR Cirrus cloud fire study

Understanding the impact of cirrus clouds on the global radiation budget is essential to determining the role of clouds in the process of climate change. The ongoing Earth Radiation Budget Experiment (ERBE) is charged with measuring the global radiation balance at the top of the atmosphere. The International Satellite Cloud Climatology Project (ISCCP) is measuring global cloud amounts and properties over a time frame similar to ERBE. Specific cloud properties are absent from the ERBE program, while ISCCP lacks the broadband radiances necessary to determine the total radiation fields. Together, results from these two global programs have the potential for improving the knowledge of the relationship between cirrus clouds and the Earth radiation balance. The First ISCCP Regional Experiment (FIRE), especially its cirrus Intensive Field Observations (IFO), provides opportunities for studying radiation measurements from the ERBE taken over areas with known cirrus cloud properties. Satellite measurements taken during the IFO are used to determine the broadband radiation fields over cirrus clouds and to examine the relationship between narrowband and broadband radiances over various known scenes. The latter constitutes the link between the ERBE and the ISCCP.

Harrison, Edwin F.↗
NASA NTRS1982

Acoustic levitating apparatus for submillimeter samples

A hemispherical focusing radiator has been employed to generate ultrahigh intensity sound waves in a gaseous medium at the center of curvature of the radiator (focal point) at 75, 107, and 163 kHz. A volumetric force is produced by optimally placing a reflector in the vicinity of the focal point to levitate samples of submillimeter sizes. It has been demonstrated that a sample with a specific gravity of 19.3 can be levitated with this apparatus. The lateral positional wandering of the sample in the force well is estimated at less than 5% of the dimension of the sample size used.

Lee, M. C.↗
NASA NTRS1997

Estimating Surface Radiation Fluxes in the Arctic from TOVS Brightness Temperatures

A new method for estimating downwelling shortwave and longwave radiation fluxes in the Arctic from TOVS brightness temperatures has been developed. The method employs a neural network to bypass computationally intensive inverse and for-ward radiative transfer calculations, Results from two drifting ice camps (CEAREX, LeadEx) and from one coastal station show that downwelling fluxes can be estimated with r.m.s. errors of 20 W/sq m for longwave radiation and 35 W/sq m for shortwave radiation. Mean errors are less than 4 W/sq m and are well within the bounds required for many climate process studies.

Schweiger, A. J.↗
NASA NTRS2004

Application of Polarization to the MODIS Aerosol Retrieval Over Land

Reflectance measurements in the visible and infrared wavelengths, from the Moderate Resolution Imaging Spectroradiometer (MODIS), are used to derive aerosol optical thicknesses (AOT) and aerosol properties over land surfaces. The measured spectral reflectance is compared with lookup tables, containing theoretical reflectance calculated by radiative transfer (RT) code. Specifically, this RT code calculates top of the atmosphere (TOA) intensities based on a scalar treatment of radiation, neglecting the effects of polarization. In the red and near infrared (NIR) wavelengths the use of the scalar RT code is of sufficient accuracy to model TOA reflectance. However, in the blue, molecular and aerosol scattering dominate the TOA signal. Here, polarization effects can be large, and should be included in the lookup table derivation. Using a RT code that allows for both vector and scalar calculations, we examine the reflectance differences at the TOA, with and without polarization. We find that the differences in blue channel TOA reflectance (vector - scalar) may reach values of 0.01 or greater, depending on the sun/surface/sensor scattering geometry. Reflectance errors of this magnitude translate to AOT differences of 0.1, which is a very large error, especially when the actual AOT is low. As a result of this study, the next version of aerosol retrieval from MODIS over land will include polarization.

Levy, Robert C.↗
NASA NTRS1972

Backscatter of solar resonance radiation. I.

Calculation of the angular dependence of the intensity of solar Lyman alpha resonantly scattered from neutral interstellar hydrogen which has penetrated the solar system. A simple model which essentially neglects temperature effects, but which includes gravity, radiation pressure, photoionization and charge exchange is used for this calculation. The results are then compared with the observations. The resonant scattering of He I 584 A is also treated.

Johnson, H. E.↗