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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Sun sensing guidance system for high altitude aircraft

A sun sensing guidance system for high altitude aircraft is described. The system is characterized by a disk shaped body mounted for rotation aboard the aircraft in exposed relation to solar radiation. The system also has a plurality of mutually isolated chambers; each chamber being characterized by an opening having a photosensor disposed therein and arranged in facing relation with the opening for receiving incident solar radiation and responsively providing a voltage output. Photosensors are connected in paired relation through a bridge circuit for providing heading error signals in response to detected imbalances in intensities of solar radiation.

Reed, R. D.↗
NASA NTRS1981

Diurnal changes in reflectance factor due to Sun-row direction interactions

Over a two year period, data were collected regarding the canopies of soybeans grown in rows in planter boxes placed on a turntable in an effort to investigate changes in the spectral reflectance factor related to row direction, Sun direction, soil background, and crop development stage. Results demonstrate that the direction of rows in a soybean canopy can affect the reflectance factor of the canopy by as much as 230%. The results for the red spectral region tend to support the validity of canopy reflectance models; results for the infrared region do not.

Bauer, M. E.↗
NASA NTRS1982

Ultraviolet continuum absorption /less than about 1000 A/ above the quiet sun transition region

Lyman continuum absorption shortward of 912 A in the quiet sun solar transition region is investigated by combining spectra obtained from the Apollo Telescope Mount experiments on Skylab. The most recent atomic data are used to compute line intensities for lines that fall on both sides of the Lyman limit. Lines of O III, O IV, O V, and S IV are considered. The computed intensity ratios of most lines from O IV, O V, and S IV agree with the experimental ratios to within a factor of 2. However, the discrepancies show no apparent wavelength dependence. From this fact, it is concluded that at least part of the discrepancy between theory and observation for lines of these ions can be accounted for by uncertainties in instrumental calibration and atomic data. However, difficulties remain in reconciling observation and theory, particularly for lines of O III, and one line of S IV. The other recent results of Schmahl and Orrall (1979) are also discussed in terms of newer atomic data.

Doschek, G. A.↗
NASA NTRS1982

The acceleration of particles on the sun and in the heliosphere

Within the heliosphere, the sun, the interplanetary medium, and at least four planetary magnetospheres contain acceleration sites which produce significant fluxes of energetic particles. The detailed study of the acceleration processes which produce the different energetic particle populations in the heliosphere is of great importance for cosmic-ray astrophysics. Shock acceleration has been recognized as one of the dominant mechanisms for providing energetic particles. Because of their unique importance to cosmic rays, the present investigation is concerned with shock acceleration processes within the heliosphere. The mechanism by which particles gain energy through their interaction with shocks is briefly reviewed. Attention is given to particle acceleration in the region of the earth's bow shock, interplanetary acceleration processes, solar flare acceleration processes, and particle acceleration at the solar wind termination shock.

Mcdonald, F. B.↗
NASA NTRS1981

The spontaneous concentration of magnetic field in the photosphere of the sun

The basic physics of magnetic flux tubes in the solar photosphere is reviewed, with areas still open to conjecture pointed out. The question of the concentration of individual small flux tubes to levels of 1-2 kilogauss, when the average solar surface magnetic field is on the order of 10 gauss, by processes of twisting and the formation of flux ropes made up of tubes wound around each other is considered together with the effects of turbulence on the flux tube. Mechanisms for tube compression by the evacuation of the gas contained within a flux tube are then examined, and the possibility of field concentration through the cooling of the gas within the tube in a superadiabatic process is suggested. Attention is then given to possible mechanisms serving to maintain the concentration of flux tubes far below the surface of the sun which gives rise to sunspots and pores as the flux tree emerges through the surface.

Parker, E. N.↗
NASA NTRS1982

The faint young sun-climate paradox - Continental influences

We examine the various mechanisms which have been proposed to compensate for the climatic effects of a 30% increase in the solar luminosity over the past 4 1/2 billion years. Although atmospheric greenhouse effects have received most attention, other mechanisms may have played a role of comparable importance. In particular, we note that the development of continents during the past 2 1/2 billion years could have had a significant secular effect on the atmosphere-ocean heat transport system. As a result, past climates may have been less susceptible to complete freeze-over. A simple energy balance model is used to demonstrate the magnitude of this effect. Because the CO2 greenhouse effect is not the only means of compensating for solar evolution, the faint-young-sun problem should not be used to infer past levels of atmospheric CO2.

Endal, A. S.↗
NASA NTRS1981

Effect of sun and sensor geometry, canopy structure and density, and atmospheric condition on the spectral response of vegetation, with particular emphasis on across-track pointing

A computer modeling and simulation study carried out to assess the effects of various sun and sensor geometries and atmospheric conditions on the directional reflected radiance of several vegetated targets is described. Spectral responses at two wavelengths, 0.68 micron and 0.80 micron, are simulated at nine sensor zenith angles, five sensor azimuths, and nine solar zenith angles for six vegetation canopies under three atmspheric conditions. The six canopies comprise two different geometries of grass canopies at low, medium, and high leaf density. The results suggest that off-nadir viewing effects are more pronounced in the red than in the IR. However, the use of such transformations as the normalized difference index is found to reduce much of the variability seen in the bands. The magnitude of off-nadir viewing effects is found to be a function of canopy geometry.

Schnetzler, C. C.↗
NASA NTRS1981

High-sensitivity circular polarimetry of the Sun, 0.5 - 1.7 microns

Using an isometric (symmetrical) photoelastic-modulator polarimeter and a Cassegrain telescope, broadband polarization measures of the Sun were obtained with unprecedented sensitivity. With an instrumental background of x 10 to the minus 6th power differential effects delta q = V/I of 1 x 10 to the minus 7th power were detectable. Some results: (1) the lambda dependence of broadband circular polarization in spots was observed out to 1.7 mu, extending the visible-light measurements of Illing, Landman, and Mickey. The q drops to of the order of 1 x 10 to the minus 5 power at 1.2 mu (in a strong spot), then rises at 1.6 mu. While molecules or other special mechanisms must play a role in the visible-light polarization, at lambda 1.5 mu the q values seem consistent with gray magneto-opacity. (2) By surveying inactive regions upper limits are set to broadband polarization due to global or deep-seated magnetic fields. For a double-toroidal pattern a differential measure is found, q sub b = (q sub ne + q sub sw - q sub nw - q sub - se) 14, at 1.7 mu, of (+1.2 + or = 0.7) x 10 to the minus 7th power a mean from three series in 1981. Based on a simple estimate by L. Biermann this would correspond to deep-down toroidal fields of or approx 2000 gauss. Extension of this work are discussed.

Kemp, J. C.↗
NASA NTRS1982

Thermal response of solar receiver aperture plates during sun walk-off

The tracking mechanism for a point-focusing concentrator may be subject to failure. If this should occur, the solar image will travel across the aperture plate, and it may also impinge on the adjacent support structure. Such an event is called 'sun walk-off'. The present investigation is concerned with the transient response of different aperture plate materials to the intense heating produced in a typical walk-off situation for parabolic dish concentrators. Receivers for two solar module systems are considered, including a high-temperature receiver that utilizes a 2-milliradian (mrad) concentrator, and a lower-temperature receiver which is coupled with a 4-mrad concentrator. It is found that during a walk-off situation the solar image travels in a straight line in the radial direction. The results obtained for a copper aperture plate were disappointing. It appears that passive metallic plates without cooling or other protective support cannot withstand the intense heating.

Wen, L.↗
NASA NTRS1983

Accelerometer-enhanced orbit control near the sun-earth L1 libration point

Because the halo-orbit about the sun-earth L(1) libration point in which the satellite ISEE-3 was maintained for nearly four years is unstable, a loose control scheme about a precomputed nominal path was implemented which required orbit maneuvers approximately every three months. Execution errors were minimized by processing on-board accelerometer telemetry data in real time, and adjusting the maneuvers. Because spacecraft vibrations caused oscillations in the accelerometer data, smoothing techniques were applied to provide accurate estimates of the performance of pulse mode thrusters employed in the spin stabilization of the spacecraft. It has been found that the processed accelerometer data has an average error of only + or - 1.3 per cent.

Muhonen, D. P.↗
NASA NTRS1982

Magnetic field-related heating instabilities in the surface layers of the sun and stars

The stability of a magnetized low-density plasma to current-driven filamentation instabilities is investigated and the results are applied to the surface layers of stars. Unlike previous studies, the initial (i.e., precoronal) state of the stellar surface atmosphere is taken to be a low-density, optically thin magnetized plasma in radiative equilibrium. The linear analysis shows that the surface layers of main-sequence stars (including the sun) which are threaded by magnetic fields are unstable; the instabilities considered lead to structuring perpendicular to the ambient magnetic fields. These results suggest that relatively modest surface motions, in conjunction with the presence of magnetic fields, suffice to account for the presence of inhomogeneous chromospheric and coronal plasma overlying a star's surface.

Ferrari, A.↗
NASA NTRS1983

The rotational spectrum of g-modes in the sun

Observations and calculations are presented which strongly suggest that the unique sequence of rotation rates due to g-modes is active in the sun. It is found that all angular harmonics from 1 = 2-11 appear to be resolved, while higher harmonics up to at least 1 = 20 also seem to be active. The extreme narrowness of many of the spectral lines indicates that the g-modes are locked in sets of constant 1 by a local mechanism which excites the modes and possibly regulates the long-term precision of their rotation. These sets act as a system of resonant, or nearly resonant, clocks which impose a degree of periodicity upon solar activity and measure the mean solar interior rotation

Wolff, C. L.↗
NASA NTRS1982

The faint young sun-climate paradox - Volcanic influences

It has been suggested that the early earth may have frozen over as a result of a fainter early sun (see Ulrich, 1975). If this had happened, climate models suggest the earth would have remained frozen through the present epoch and into the distant future. We suggest that volcanic influences could allow a passage from the frozen branch into the unfrozen branch of climate models should conditions on earth be suitable for the latter climate change. A broad equatorial belt of volcanic ash is one scenario which would allow a transfer from the frozen earth state into the unfrozen one.

Schatten, K. H.↗
NASA NTRS1983

Outflow from the sun's polar corona

New observations of systematic Doppler shifts of EUV resonance lines formed both in the low corona and transition region are reported. They were made with an improved, high-resolution, stable, rocket-borne spectrometer flown on 1981 November 23. The chord of the solar disk scanned by the spectrometer crossed the north polar cap hole and its low-latitude extension, and also a compact low-latitude hole near sun center. Within both holes, the lines were systematically shifted to shorter wavelengths relative to the rest of the solar disk. These observations strengthen the association of negative Doppler shifts with coronal holes and indicate that this 'blueshift signature', previously observed only within small low-latitude holes, is also characteristic of the low-density polar corona. The mean relative blueshift measured in lambda 625 Mg X (T = 10 to the 6.15 power K) within the polar hole was about 8 km/s.

Orrall, F. Q.↗
NASA NTRS1983

The sun at 1.4 GHz - Intensity and polarization

1.4 GHz radio pictures of the sun are presented which were made on 1981 September 26 using the VLA with a resolution of 40 arcsec. Features observed include active regions, limb brightening, coronal holes and filament channels; all correspond well with features seen in H-alpha or He 10830 A, and can be explained as relative enhancements or depressions of free-free brmsstrahlung from the corona and upper transition region. The degree of circular polarization of the radiation from active regions ranges up to 0.2 and the sense corresponds, with a few exceptions, with the polarity of the photospheric magnetograms, even though the 1.4 GHz radiation arises from the corona at a height of 10 to 50 Mm. The polarization is in the sense of the x-mode and is highest near the edges of active regions, whereas the brightness is highest (maximum of 2.2 x 10 to the 6th K) near the line of zero polarization.

Dulk, G. A.↗
NASA NTRS1983

H I Lyman-alpha in the sun - The effects of partial redistribution in the line wings

A form for the Lyman-alpha redistribution function that is modified to include the effects of collisional coupling between different energy states is presented. It is noted that this extends the work of Yelnik et al. (1981) and Roussel-Dupre(1982). By separating the source function into its primary components at all heights, the formation of Lyman-alpha radiation is investigated in detail. Frequencies in the line wing can be mapped back in such a way that the heights of formation for a change in lambda of plus or minus 0.5-6 A span the entire solar chromosphere. The usefulness of the Lyman-alpha wing as an atmospheric probe, however, is limited by long thermalization lengths, which result from almost coherent scattering outside of the Doppler core. When the line profiles calculated from the chromospheric models of Vernazza et al. (1981) are compared with the quiet-sun observations of Roussel-Dupre, agreement is found in integrated core intensities and in the line wing shape and normalized center-to-limb behavior. The calculated wing intensities, however, are a factor of four too small.

Roussel-Dupre, D.↗
NASA NTRS1983

The young sun and the atmosphere and photochemistry of the early earth

The origin and evolution of the earth's early atmosphere depend crucially on the dissipation time of the primitive solar nebula (SN). Using different theories of turbulence, the dissipation time of an SN of 0.1 solar mass is estimated as 2.5-8.3 Myr. Because accretion times are usually much longer, it is concluded that most planetary accretion must have occurred in a gas-free environment. Using new IUE data, a wavelength-dependent UV flux is constructed for the young sun which is then used to study the photochemistry and concentrations of O, O2, O3, OH, H, HCO and formaldehyde H2CO in the earth's early prebiological atmosphere.

Canuto, V. M.↗
NASA NTRS1984

Passive Sun seeker/tracker and a thermally activated power module

Development and testing of two mechanisms using a shape memory alloy metal (NITINOL) as the power source are described. The two mechanisms developed are a passive Sun Seeker/Tracker and a generic type power module. These mechanisms use NITINOL wire initially strained in pure torsion which provides the greatest mechanical work capacity upon recovery, as compared to other deformation modes (i.e., tension, helical springs, and bending).

Siebert, C. J.↗