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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

The detection of global convection on the sun by an analysis of line shift data of the John M. Wilcox Solar Observatory at Stanford University

Signatures of the existence of the global convection in the sun were found in the absorption line shift data of the John M. Wilcox Solar Observatory at Stanford University. The signatures are characterized by persistent periodic time variations in the east-west component of the velocity fields defined by fitting a slope to the line shift data in a certain longitude window at a specified latitude and longitude by a least square method. The variations indicate that the amplitude of the velocity fields is about 100 m/s. It is suggested that several modes of global convection are coexisting in the solar convection zone.

Yoshimura, Hirokazu↗
NASA NTRS1988

Comment on 'Highly relativistic magnetospheric electrons: A role in coupling to the middle atmosphere?'

It is argued that Baker et al. (1987) may have underestimated the electron precipitation rate in the southern hemisphere in the region extending west to about 90 deg in longitude from the southern extension of the center of the South Atlantic geomagnetic anomaly (SAGA). In a reply, it is noted that Baker et al. restricted their study to the case of uniform precipitation in latitude and longitude throughout the outer-zone area, thus entailing a great simplification of the real-world situation. Comments are made on the effects of the SAGA on electron precipitation.

Sheldon, W. R.↗
NASA NTRS1989

The sunspot cycle variations of the neutral line on the source surface

The earlier presentation method of the sunspot cycle variations of the neutral line on the source surface by introducing the view longitude is defined. It is shown that the neutral line seen from the view longitude and the equivalent dipole (determined by Hoeksema, 1984) showed a trend of rotation throughout the sunspot cycle. However, the surface bounded by the neutral line is generally far from a circle. In fact, our combined presentation of both the equivalnet dipole and the nuetral line indicates graphically that such a simple description of the rotating (equivalent) dipole may be misleading. The concept of an inclined dipole with respect to the rotation axis may also be misleading.

Saito, T.↗
NASA NTRS1989

Three-dimensional linear instability modeling of the cloud level Venus atmosphere

Based on the success of several two-dimensional (latitude, longitude) linear barotropic instability models at matching some of the observed characteristics of the cloud level, polar region of the Venus atmosphere, a more realistic, linear, three-dimensional (height, latitude and longitude) model has been developed to further test the hypothesis that the observed features can be described by linear instability theory. The approach taken is to vary the model input parameters to see whether it is possible to produce modes that resemble the observations of wave activity and to compare those input parameters with other observations of the mean state. Sensitivity studies show that in addition to a well-documented dependence on the mean zonal wind, the growth and propagation of unstable modes depends on the latitude variation of the mean temperature (and hence static stability). These studies have led to the specification of a model basic state wind and temperature field that produces modes which are matched to observations of spatial structure, preferred wavenumber and phase speed of the polar disturbances. Wavenumber 2 is found to have the shortest growth time and unlike the two-dimensional results, wavenumbers 1-3 share a nearly common period of about 3 days. The derived basic state has a temperature field that is quite similar to Pioneer Venus observations; however, in some regions the model basic state wind field departs from cyclostrophic values based on temperature observations.

Elson, Lee S.↗
NASA NTRS1990

A midlatitude interhemispheric model of the O(+) (2P) airglow emission at 7320 A

The results are reported of the first interhemispheric model of the 7320 A airglow covering mid- and low latitudes, at all longitudes. A comprehensive model of the ionosphere and thermosphere is used to compute volume emission rates as a function of altitude, latitude, longitude, and local time. Selected results are shown here to illustrate the computational capability. In particular, the diurnal and seasonal variability and interhemispheric coupling of conjugate photoelectrons are discussed. The model is particularly well suited for airglow studies, and provides a valuable tool for the comparison, interpretation, and planning of spectroscopic observations made from orbiting platforms for all significant thermospheric emissions.

Torr, Marsha R.↗
NASA NTRS1990

The X-ray orbital lightcurve of TY Pyx - A 3.2-day eclipsing RS CVn system

An Exosat observation of one orbital cycle of the eclipsing 3.2-d RS CVn system TY Pyx is reported. This system contains two almost identical GV stars, which is unusual because most RS CVns contain F or G main-sequence star and a K subgiant. The X-ray light curves of the primary and secondary eclipses are different above and below 1 keV. At lower energies a sharp eclipse is seen, but at higher energies it is absent. This confirms earlier observations that the 1.5-4.0 x 10 to the 7th K emission from the RS CVn systems is not eclipsed and comes from an extended region, whereas the 7 x 10 to the 6th K emission comes from more compact solarlike active regions. Modeling the X-ray eclipse detected at low energies requires two emission regions, one on each star, covering less than 20 percent of the stellar surface. To avoid a strong orbital modulation caused by both regions being either simultaneously visible, or not visible, they must be located 180 deg apart in longitude. The derived longitudes are 0 and 180 deg; i.e., they point at each other on the inward faces of the two stars.

Culhane, J. L.↗
NASA NTRS1990

Sea level oscillations observed with Geosat along the two shear fronts of the Pacific north equatorial countercurrent

Geosat data are processed and analyzed into sea-level variations over the tropical Pacific Ocean during the first 26 months of the Exact Repeat Mission from November 15, 1986 to January 15, 1989. The resolution is 1 deg latitude and longitude and 10 days. The frequency and wavenumber spectra of these sea-level variations show much more energy in the northeastern than in the western or southern Pacific. Energy highs of 8000 sq cm per (cycle per day) per (cycle per deg longitude) are clearly identified along 5 deg N for periods of 28-40 days and wavelengths of 1000-2200 km (band 1), and along 12 deg N for periods of 50-90 days and wavelengths of 630-950 km (band 2).

Perigaud, Claire↗
NASA NTRS1990

Small scale structure in the brightness of the zodiacal light - Ground-based observations

Ground-based observations of the evening zodiacal light are used to derive a table of zodiacal light brightnesses at spatial resolutions as small as 0.5 deg in differential ecliptic longitude and 1.0 deg in ecliptic latitude. Significant differences are found in the brightness distribution above and below the ecliptic plane. Attention is given to a feature found in the brightness at all ecliptic latitudes, between differential ecliptic longitudes 39 deg and 42 deg.

Misconi, Nebil Y.↗
NASA NTRS1990

Long-term study of longitudinal dependence in primary particle precipitation in the north Jovian aurora

The wavelength-dependent absorption apparent in IUE spectra of the north Jovian aurora is analyzed to determine the column density of hydrocarbons above the altitude of the FUV auroral emission. Both the magnetotail and torus auroral zone models are considered in estimating zenith angles, with very similar results obtained for both models. It is found that the hydrocarbon column density above the FUV emission displays a consistent dependence on magnetic longitude, with the peak density occurring approximately coincident with the peak in the observed auroral intensity. Two distinct scenarios for the longitude dependence of the column density are discussed. In one, the Jovian upper atmosphere is longitudinally homogeneous, and the variation in optical depth is due to a variation in penetration, and thus energy, of the primary particles. In the other, the energy of the primaries is longitudinally homogeneous, and it is aeronomic properties which change, probably due to auroral heating.

Livengood, T. A.↗
NASA NTRS1990

TOPEX/POSEIDON orbit maintenance maneuver design

The Ocean Topography Experiment (TOPEX/POSEIDON) mission orbit requirements are outlined, as well as its control and maneuver spacing requirements including longitude and time targeting. A ground-track prediction model dealing with geopotential, luni-solar gravity, and atmospheric-drag perturbations is considered. Targeting with all modeled perturbations is discussed, and such ground-track prediction errors as initial semimajor axis, orbit-determination, maneuver-execution, and atmospheric-density modeling errors are assessed. A longitude targeting strategy for two extreme situations is investigated employing all modeled perturbations and prediction errors. It is concluded that atmospheric-drag modeling errors are the prevailing ground-track prediction error source early in the mission during high solar flux, and that low solar-flux levels expected late in the experiment stipulate smaller maneuver magnitudes.

Bhat, R. S.↗
NASA NTRS1990

Aurora and airglow on the night side of Neptune

The latitude-longitude distribution of emissions detected by the Voyager ultraviolet spectrometer on the dark hemisphere of Neptune have been examined. The emissions have two significant geographic features: (1) a broad peak near longitude 60 deg W that extends rather uniformly over the entire range of observed latitudes (55 deg S to 50 deg N); and (2) a brighter, narrower peak near the south pole and 240 deg W. The first peak is interpreted as due to excitation of the night side atmosphere by photoelectrons from the magnetically conjugate, sunlit atmosphere. The second peak can plausibly be attributed to a southern aurora; the field geometry would then seem to require a conjugate (and probably brighter) northern aurora that escaped detection poleward of the latitude range sampled by the UVS data. The power for such an aurora could be extracted from Neptune's rotation by the injection of plasma at Triton's orbit at a rate dm/dt of about 1 kg/s.

Sandel, B. R.↗
NASA NTRS1990

Observations of the earth's polar cleft at large radial distances with the Hawkeye 1 magnetometer

Based on 364 spacecraft passes through the dayside region, the position of the polar cleft at large radial distances was determined with the magnetometer flown on Hawkeye 1. This data set is ideal for the study of the cusp and cleft region. Identification of the cleft depended on noting strong negative deviations of the magnetic field strength in the region from that of the dipole field. In solar magnetic coordinates, cleft observations were found between 40 deg and 70 deg latitude and + or - 75 deg latitude and + or - 75 deg longitude, while in geocentricmagnetospheric coordinates, these observations were found between 20 deg and 75 deg latitude and + or - 75 deg longitude. The extreme longitudinal extent of 150 deg is larger than those reported in some previous studies. Large magnetic depressions associated with the cleft extend out to 12 earth radii.

Farrell, W. M.↗
NASA NTRS1991

Zodiacal emission. II - Dust near ecliptic

The location and brightness of the peak zodiacal emission are derived from IRAS observations of the diffuse infrared background radiation. A uniform ellipsoid geometrical model is fitted to the latitudes of peak brightness, from which the orientation of the surface of maximum dust density near the earth's orbit is derived. Deviations from the uniform ellipsoid indicate that this surface bends toward the invariable plane of the solar system beyond 2.5 AU from the sun. The variations of the peak zodiacal emission brightness with solar elongation and observation date allow two independent determinations of the gradient of the volume emissivity of interplanetary dust. Combined with theoretical calculations of the infrared emission from silicate and graphite grains heated by the sun, the gradient of the dust density is found to be n proportional to r exp (-1.1 + or - 0.2). Systematic brightness variations with ecliptic longitude reveal two potential asymmetries in the dust complex: excess emission following the earth in its orbit, and excess emission along lines of sight with ecliptic longitude lambda = 190 + or - 20 deg and solar lambda about 100 deg.

Reach, William T.↗
NASA NTRS1991

A 21 centimeter line survey of a region around four outer galaxy open clusters

To investigate the role of atomic gas in the interaction of massive stars with the interstellar medium, a 21 cm survey was made of a region surrounding four young, outer Galaxy open clusters, NGC 281, 433, 436, and 457. A rectangular area ranging in Galactic longitude from 115 to 133 deg, and in latitude from -10 to -1 deg, was observed with the NRAO 140 foot telescope. Most of this region was fully sampled, resulting in spatial resolution equal to the telescope's 21 arcmin beamwidth. The data are presented as longitude-latitude and latitude-velocity contour maps. A catalog of atomic gas features (e.g., clouds, filaments, and shells) is given.

Leisawitz, David↗
NASA NTRS1991

Ground-based infrared measurements of HNO3 total column abundances - Long-term trend and variability

The long-term trend and variability of the total column amount of atmospheric nitric acid (HNO3) have been investigated. The study was based on time series of infrared solar absorption spectra recorded at two sites, the International Scientific Station of the Jungfraujoch (ISSJ) in the Swiss Alps (altitude 3.6 km, latitude 46.5 N, longitude 8.0 E) and the National Solar Observatory McMath solar telescope facility on Kitt Peak (altitude 2.1 km, latitude 31.9 N, longitude 111.6 W), Arizona. Measurements of HNO3 absorption features recorded at Kitt Peak between 1980 and 1990 and measurements of the same features in ISSJ solar spectra obtained in June 1951 and June 1986 to June 1990 provide quantitative estimates of the long-term trend in the HNO3 total vertical column. It is found that the trend deduced for each site is equal to zero within the measurement uncertainty. The modern ISSJ measurements show that the fitted trend is (-0.16 + or - 0.50) percent/yr, 2 sigma, which indicates that there has been no detectable change in HNO3 total column over three decades. The Kitt Peak measurements show marked variability in the HNO3 total column, but no obvious seasonal cycle is observed.

Rinsland, C. P.↗
NASA NTRS1991

Tectonic evolution of Lavinia Planitia, Venus

High resolution radar images from the Magellan spacecraft have revealed the first details of the morphology of the Lavinia Planitia region of Venus. Lavinia is a broad lowland over 2000 km across, centered at about 45 deg S latitude, 345 deg E longitude. Herein, the tectonic evolution of Lavinia is discussed, and its possible relationship to processes operating in the planet's interior. The discussion is restricted to the region from 37.3 to 52.6 deg S latitude and from about 340 to 0 deg E longitude. One of the most interesting characteristics of Lavinia is that the entire region possesses a regional tectonic framework of striking regularity. Lavinia is also transected by a complex pattern of belts of intense tectonic deformation known as ridge belts. Despite the gross topographic similarity of all of the ridge belts in Lavinia, they exhibit two rather distinct styles of near surface deformation. One is composed of sets of broad, arch-like ridges rising above the surrounding plains. In the other type, obvious fold-like ridges are rare to absent in the radar images. Both type show evidence for small amounts of shear distributed across the belts.

Squyres, Steven W.↗
NASA NTRS1991

The control network of Mars: April 1991

The modern geodetic control network of Mars was first established based on Mariner 9 images with 1-2 km/pixel resolutions and covered almost the entire Martian surface. The introduction of higher resolution (10-200 meter/pixel) Viking Orbiter images greatly improved the accuracy and density of points in the control network. Analysis of the Viking Lander radio tracking data led to more accurate measurements of Mars' rotation period, spin axis direction, and the lander coordinates relative to the inertial reference frame. The prime meridian on Mars was defined by the Geodesy/Cartography Group of the Mariner 9 Television Team as the crater Airy-0, located about 5 degrees south of the equator. The Viking 1 Lander site was identified on a high resolution Viking frame. The control point measurements form the basis of a least squares solution determined by analytical triangulation after the pixel measurements are corrected for geometric distortions and converted to millimeter coordinates in the camera focal plane. Photogrammetric strips encircling Mars at the equator and at 60 degree north south were used to strengthen the overall net and improve the accuracy of the coordinates of points. In addition, photogrammetric strips along 0, 90, 180, and 270 degrees longitude to the Viking 1 Lander site have all significantly strengthened the control network. Most recently, photogrammetric strips were added to the net along 30 degrees north latitude between 0 and 180 degrees, and along 30 degrees between 180 and 360 degrees. The Viking 1 Lander site and Airy-0 are linked through photogrammetric strips occurring along the 0 degree meridian from Airy-0 to 65 degrees north, from that point through the Viking 1 Lander site to the equator, and along the equator to 180 degrees longitude. The Viking 1 lander site is thus a well calibrated area with coordinates of points accurate to approximately 200 meters relative to the J2000 inertial coordinate system. This will be a useful calibration region for upcoming missions. The current status of the control network calculations is presented.

Davies, Merton E.↗
NASA NTRS1991

Mars elevation distribution

A Digital Terrain Model (DTM) of Mars was derived with both Mercator and Sinusoidal Equal-Area projections from the global topographic map of Mars (scale 1:15 million, contour interval 1 km). Elevations on the map are referred to Mars' topographic datum that is defined by the gravity field at a 6.1-millibar pressure surface with respect to the center of mass of Mars. The DTM has a resolution at the equator of 1/59.226 degrees (exactly 1 km) per pixel. By using the DTM, the volumetric distribution of Mars topography above and below the datum has previously been calculated. Three types of elevation distributions of Mars' topography were calculated from the same DTM: (1) the frequency distribution of elevations at the pixel resolution; (2) average elevations in increments of 6 degrees in both longitude and latitude; and (3) average elevations in 36 separate blocks, each covering 30 degrees of latitude and 60 degrees of longitude.

Wu, Sherman S. C.↗