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At least 703 records · Page 39

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NASA NTRS1988

Structure of magnetic fields on the quiet sun

To obtain quantitative temporal and spatial information on the network magnetic fields, auto- and cross-correlation techniques are applied to the Big Bear videomagnetogram data. The average size of the network magnetic elements derived from the auto-correlation curve is about 5700 km. The distance between the primary and secondary peak in the auto-correlation curve is about 17,000 km, which is half of the size of the supergranule as determined from the velocity map. The canceling features and the emergence of ephemeral regions are the major sources for the loss and replenishment of magnetic flux on the quiet sun.

Wang, Haimin↗

NASA NTRS1989

Application of the Langley plot for calibration of sun sensors for the Halogen Occultation Experiment (HALOE)

The calibration of the Halogen Occultation Experiment (HALOE) sun sensor is described. This system consists of two energy-balancing silicon detectors which provide coarse azimuth and elevation control signals and a silicon photodiode array which provides top and bottom solar edge data for fine elevation control. All three detectors were calibrated on a mountaintop near Tucson, Ariz., using the Langley plot technique. The conventional Langley plot technique was modified to allow calibration of the two coarse detectors, which operate wideband. A brief description of the test setup is given. The HALOE instrument is a gas correlation radiometer that is now being developed for the Upper Atmospheric Research Satellite.

Moore, Alvah S., Jr.↗

NASA NTRS1988

Estimation of surface insolation using sun-synchronous satellite data

A technique is presented for estimating insolation at the earth's surface using only sun-synchronous satellite data. The technique was tested by comparing the insolation results from year-long satellite data sets with simultaneous ground-measured insolation taken at five continental United States sites. Monthly average insolation values derived from the satellite data showed a standard error of 4.2 W/sq m, or 2.7 percent of the average ground insolation value.

Darnell, Wayne L.↗

NASA NTRS1989

A parameterization for longwave surface radiation from sun-synchronous satellite data

A parameterization is presented for computing downward, upward, and net longwave radiation at the earth's surface using data from NOAA sun-synchronous satellites. The parameterization is applied to satellite soundings for April, 1982 over a large region of the tropical Pacific Ocean. Sensitivity studies were used to estimate the random and systematic errors in computed fluxes due to probable errors in TOVS-derived parameters. It is suggested that large biases in the results due to errors in TOVS-derived parameters may be corrected with data from the International Satellite Cloud Climatology Project.

Gupta, Shashi K.↗

NASA NTRS1989

Magnetic flux transport on the sun

Although most of the magnetic flux observed on the sun originates in the low-latitude sunspot belts, this flux is gradually dispersed over a much wider range of latitudes by supergranular convective motions and meridional circulation. Numerical simulations show how these transport processes interact over the 11-year sunspot cycle to produce a strong 'topknot' polar field, whose existence near sunspot minimum is suggested by the observed strength of the interplanetary magnetic field and by the observed areal extent of polar coronal holes. The required rates of diffusion and flow are consistent with the decay rates of active regions and with the rotational properties of the large-scale solar magnetic field.

Wang, Y.-M.↗

NASA NTRS1989

The nonlinear breakup of the sun's toroidal field

There are good reasons for believing that the sun has a strong toroidal magnetic field in the stably stratified region of convective overshoot sandwiched between the radiative zone and convective zone proper. The magnetic field in this region is modeled by studying the behavior of a layer of uniform field embedded in a subadiabatic atmosphere. Since the field can support extra mass, such a configuration is top-heavy, and instabilities of the Rayleigh-Taylor type can occur. Numerical integration of the two-dimensional compressible MHD equations makes it possible to follow the evolution of this instability into the nonlinear regime. The initial buoyancy-driven instability of the magnetic field gives rise to strong shearing motions, thereby exciting secondary Kelvin-Helmholtz instabilities which wrap the gas into regions of intense vorticity. The somewhat surprising subsequent motions are determined primarily by the strong interactions between vortices.

Hughes, D. W.↗

NASA NTRS1990

Lower solar chromosphere-corona transition region. III - Implications of the observed quiet-sun emission measure including wave pressure effects

The observed form of the emission measure (EM) is used as a function of temperature to infer the wave energy flux density and pressure throughout the lower transition region (TR). This procedure eliminates the need for specifying how the wave energy flux density is damped and addresses the question of whether there is any form of the mechanical heating associated with the degradation of an upward traveling wave energy flux density which is consistent with the observed EM and other observational constraints for the quiet sun. It is found that the observed form of the EM curve is incompatible with waves traveling vertically at the sound speed, regardless of any filling factor arguments. The same conclusion also applies to waves traveling at the Alfven speed, unless it is assumed that the emission in lower TR lines originates solely from small, spatially unresolved regions of large magnetic field strength (100 G), which cover a small fraction (filling factors of 1 percent) of the solar surface.

Woods, D. Tod↗

NASA NTRS1990

The systematic radial downflow in the transition region of the quiet sun from limb-to-limb observations of the C IV resonance lines

This paper presents absolute velocities of C IV 1548, 1550 nm measured as a function of position along the solar equator, which was free of both active regions and coronal holes, and uniformly representative of the quiet sun. These observations were made with moderate spatial resolution (18 arcsec) using an EUV spectrometer dedicated to measuring absolute wavelengths (velocities) by direct comparison with a platinum spectrum generated on board the sounding rocket. On the assumption that systematic horizontal motions cancel statistically so that the line-of-sight velocities approach zero at the limb, a net radial downflow of 7.5 + or - 1.0 km/sec was found. The assumption was tested using the wavelength reference and found to be valid within the absolute accuracy of the rest wavelengths of the C IV lines.

Rottman, Gary J.↗

NASA NTRS1990

Period and phase of the 88-year solar cycle and the Maunder minimum - Evidence for a chaotic sun

The problem of whether the solar dynamo is quasi-periodic or chaotic is addressed by examining 1500 years of sunspot, geomagnetic, and auroral activity cycles. Sub-harmonics were found of the fundamental solar cycle period during the years preceding the Maunder minimum and loss of phase of the subharmonic on emergence from it. These phenomena are indicative of chaos. They indicate that the solar dynamo is chaotic and is operating in a region close to the transition between period doubling and chaos. Since Maunder-type minima reoccur irregularly for millennia, it appears that the sun remains close to this transition to and from chaos. This is postulated to be a universal characteristic of solar type stars caused by feedback in the dynamo number.

Feynman, J.↗

NASA NTRS1990

Relationship Between Rainfall in the Northern Hemisphere and Impulses of the Torque in the Sun's Motion

The analysis of major change in the angular momentum of the sun's irregular motion about the barycenter of the solar system, represented by extrema in the running variance of impulses of the torque (IOT), discloses a connection with both extrema in the Gleissberg cycle of secular sunspot activity and maxima in the thickness of varves from Lake Saki, Crimea. This significant relationship can be traced back to the 7th century. Further inquiries link the running variance in IOT to rainfall over central Europe, England, Wales, eastern United States, and India, as well as to temperature in Europe. This significant correlation covers more than 130 years.

Landscheidt, T.↗

NASA NTRS1990

A Review and Reflections on the Sun-Climate Connection

The field of sun-climate is beset with an extraordinary number of numerical correlations attempting to relate various periodicities of solar activity with changes in the Earth's weather and climate. Signatures representing climatological variability have been sought for cycles as short as the solar 28-day rotational period up to Milankovich periods of thousands of years, although a majority of correlations have concentrated on the 11-year sunspot and 22-year Hale double sunspot cycles. For the shorter term, parameters including temperature, pressure, winds storm tracks, rainfall, and water levels in rivers and lakes, etc. have been correlated with solar variability. For longer periods, it has been necessary to seek more indirect evidence in ice cores, tree rings, and geologic deep sea cores. Other atmospheric parameters relating to atmospheric electricity and the global electric circuit have also been correlated in similar fashion. Unfortunately, few, if any, of this wide spectrum of numerical correlations have been associated with any viable physical explanation, making most studies in the field an exercise in numerical statistics. More recently, a few suggestions for plausible coupling processes have begun to appear. These, coupled with new and stronger correlations involving selective binning of climatological data sets have injected new life and hope to this field. An overview is given of the historical past and current perspectives, to evaluate possible avenues for defining physical linking processes in the future.

Goldberg, Richard A.↗

NASA NTRS1991

Sublimation rates of carbon monoxide and carbon dioxide from comet nuclei at large distances from the Sun

One of the more attractive among the plausible scenarios for the major emission event recently observed on Comet Halley at a heliocentric distance of 14.3 AU is activation of a source of ejecta driven by an icy substance much more volatile than water. As prerequisite for the forthcoming detailed analysis of the imaging observations of this event, a simple model is proposed that yields the sublimation rate versus time at any location on the surface of a rotating cometary nucleus for two candidate ices: carbon monoxide and carbon dioxide. The model's variable parameters are the comet's heliocentric distance r and the Sun's instantaneous zenith angle z.

Sekanina, Zdenek↗

NASA NTRS1990

Sun-earth libration point trajectories that avoid the solar exclusion zone

Three-dimensional orbits in the restricted three-body problem have been the subject of a number of recent studies. One type of three-dimensional, quasi-periodic orbit that emanates from the general vicinity of the collinear libration points is known as a 'Lissajous' trajectory. Future mission plans include trajectories near the interior libration point. (L1) in the sun-earth system and may consider Lissajous orbits as part of the trajectory design. Such orbits may be constrained, however, to remain beyond the region of the solar disk as viewed from earth. This effort involves the numerical determination of Lissajous trajectories of arbitrary, but predetermined, length that never violate such a constraint as a result of maneuvers directed, in general, perpendicular to the ecliptic plane.

Howell, K. C.↗

NASA NTRS1991

Proposal for a sun-following moonbase

A new concept for a manned lunar facility is proposed, where the moon base moves across the moon to follow the sun. Such a facility has advantages of continuous solar power, no reduced activity period during the lunar night, and constantly new territory to sample.

Landis, G. A.↗

NASA NTRS1991

The microwave brightness temperature spectrum of the quiet sun

New measurements of the microwave brightness temperature spectrum of the center of the quiet sun, acquired at Owens Valley over several months during the 1986-1987 sunspot minimum, are reported. The resulting brightness temperature spectra are consistent with previous data, but exhibit much less frequency-to-frequency scatter. The corona is fitted well by an optically thin source at temperature of 10 to the 6th k, scale height H = 5 x 10 to the 9th, and density of 3.2 x 10 to the 8th/cu cm, and the chromosphere, an optically thick source at around 11,000 k.

Zirin, H.↗

NASA NTRS1991

The sun's luminosity over a complete solar cycle

The Active Cavity Radiometer Irradiance Monitor (ACRIM I) measured the sun's luminosity from early 1980 to late 1989. The first account of the complete ACRIM I data set is presented and evidence is given which confirms that solar luminosity varies with the 11-yr solar cycle. This slow variation closely follows statistical measures of the distribution of magnetic and photospheric features on the solar surface. An exception to this correlation occurred in the form of a remarkable irradiance excess during 1980, at about the time of the sunspot maximum of solar cycle 21. The linkage, over a whole cycle, of luminosity variation to photospheric activity suggests the existence of an unknown physical mechanism other than the thermal diffusion model that explains luminosity deficits due to sunspots. Luminosity models connecting total irradiance to global indicators of solar activity are consistent with the gross features of the variability but fail to account for the 1980 irradiance excess.

Willson, Richard C.↗

NASA NTRS1991

Magnetic flux transport and the sun's dipole moment - New twists to the Babcock-Leighton model

The mechanisms that give rise to the sun's large-scale poloidal magnetic field are explored in the framework of the Babcock-Leighton (BL) model. It is shown that there are in general two quite distinct contributions to the generation of the 'alpha effect': the first is associated with the axial tilts of the bipolar magnetic regions as they erupt at the surface, while the second arises through the interaction between diffusion and flow as the magnetic flux is dispersed over the surface. The general relationship between flux transport and the BL dynamo is discussed.

Wang, Y.-M.↗

NASA NTRS1990

Cool stars, stellar systems, and the sun; Proceedings of the 6th Cambridge Workshop, Seattle, WA, Sept. 18-21, 1989

The present conference on cool stars, stellar systems, and the sun encompasses stellar chromospheres and coronae, binary stars, the stellar evolution of contracting stars and red giants, stellar evolution abundances of the elements, mass loss and envelopes, and stellar pulsation. Specific issues addressed include theories regarding the acoustic and magnetic heating of stellar chromospheres and coronae, stellar granulation, wave heating in magnetic flux tubes, observations of the solar Ca-II lines, longitudinal-transverse magnetic tube waves in the solar atmosphere, radio emission from rapidly rotating cool giant stars, and spot temperatures and area coverages on active dwarf stars. Also addressed are the optical and UV spectra of RS-CVn stars, emission lines from T-Tauri stars, the spectroscopy of HR1614 group stars, red giants in external galaxies, the rotation of evolved stars, the transition from red giant to planetary nebula, and radiative transfer in the dynamic atmospheres of variable stars.

Wallerstein, George↗