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At least 1,333 records · Page 74
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

25 years of the impact-volcanic controversy - Is there anything new under the sun or inside the earth?

The evidence for meteorite impact and internal volcanic processes (cryptovolcanism) as the cause of the K-T extinction and for cryptoexplosion structures is briefly reviewed. It is shown that numerous studies reinforce the view that shock-metamorphic features uniquely indicate shock pressure from 5 GPa to over 50 GPa, that no generally accepted shock-metamorphic features have been found in unquestioned volcanic structures, and that no internal process has been demonstrated to produce the required shock pressures. This history therefore supports an impact origin for both cryptoexplosion structures and the K-T event. Future discoveries required to make the cryptovolcanic theory a respectable alternative are discussed.

French, Bevan M.↗
NASA NTRS1990

Millimeter and microwave activity of the sun

Preliminary results of high-spatial-resolution millimeter observations of recent solar flares carried out with the Berkeley-Illinois-Maryland Array are presented. Findings obtained from multifrequency observations using the VLA are reported. The time profiles of several bursts during March 1989 are presented. Most of the bursts were impulsive and short-lived, and a number of longer-duration bursts were observed as well. All the bursts were fairly weak, with none exceeding 1 sfu of correlated amplitude. A number had rise times less than the available resolution of 10 sec. VLA observations in September 1988 showed that there is exceptionally good correspondence between the 1.5-GHz radio sources and the regions which are dark in He 10830 images and bright in Ca images.

Kundu, M. R.↗
NASA NTRS1990

The sun at the VLA's metric and decimetric wavelengths

Preliminary results of solar observations at 0.333 and 1.5 GHz made with the VLA during the September 11-17, 1988 period are presented. Generally, there are few structural changes in the active region sources from one day to the next, suggesting that structural evolution is relatively slow. Contour maps at 1.5 GHz are presented for each of the four days. Two noise storms were present at 0.33 GHz all week and were highly polarized.

White, S. M.↗
NASA NTRS1992

Helium diffusion in the sun

We calculate improved standard solar models using the new Livermore (OPAL) opacity tables, an accurate (exportable) nuclear energy generation routine which takes account of recent measurements and analyses, and the recent Anders-Grevesse determination of heavy element abundances. We also evaluate directly the effect of the diffusion of helium with respect to hydrogen on the calculated neutrino fluxes, on the primordial solar helium abundance, and on the depth of the convective zone. Helium diffusion increases the predicted event rates by about 0.8 SNU, or 11 percent of the total rate, in the chlorine solar neutrino experiment, by about 3.5 SNU, or 3 percent, in the gallium solar neutrino experiments, and by about 12 percent in the Kamiokande and SNO solar neutrino experiments. The best standard solar model including helium diffusion and the most accurate nuclear parameters, element abundances, and radiative opacity predicts a value of 8.0 SNU +/- 3.0 SNU for the C1-37 experiment and 132 +21/-17 SNU for the Ga - 71 experiment, where the uncertainties include 3 sigma errors for all measured input parameters.

Bahcall, J. N.↗
NASA NTRS1993

Imaging the sun in hard x rays using Fourier telescopes

For several years, solar flares have been observed with a variety of instruments confirming that tremendous amounts of energy are locally stored in the solar magnetic field and then rapidly released during the life of the flare. In concert with observations, theorists have attempted to describe the means by which these energetic events occur and evolve. Two competing theories have emerged and have stood the test of time. One theory describes the flare in terms of nonthermal, electron beam injection into a thick target while the other uses a thermal approach. Both theories provide results which are reasonably consistent with current observations; but to date, none have been able to provide conclusive evidence as to the validity of either model. Imaging on short time scales (1 s) and/or small size scales (1 arc s) should give definitive answers to these questions. In order to test whether a realistic telescope can indeed discriminate between models, we construct model sources based upon the thermal and the nonthermal models and calculate the emission as a function of time and energy in the range from 10 to 100 keV. In addition, we construct model telescopes representing both the spatial modulation collimator (SMC) and the rotating modulation collimator (RMC) techniques of observation using random photon counting statistics. With these two types of telescopes we numerically simulate the instrument response to the above two model flares to see if there are distinct x-ray signatures which may be discernable. We find that theoretical descriptions of the primary models of solar flares do indeed predict different hard x-ray signatures for 1 sec time scales and at 1-5 arc sec spatial resolution. However, these distinguishing signatures can best be observed early in the impulsive phase and from a position perpendicular to the plane of the loop. Furthermore, we find that Fourier telescopes with reasonable and currently attainable design characteristics can image these signatures and that the same sensitive areas and short temporal integration times relative to source evolution (i.e., 1 s), the RMC and the SMC will both provide about the same performance.

Campbell, J. W.↗
NASA NTRS1992

Microwave spectroscopy of the active sun

In studies of solar active regions and bursts, the ability to obtain spatially resolved radio spectra (brightness temperature spectra) opens a whole new range of possibilities for study of the solar corona. For active regions, two-dimensional maps of brightness temperature over a wide range of frequencies allows one to determine temperature, column density, and magnetic field strength over the entire region in a straightforward, unambiguous way. For flares, the time-dependent electron energy distribution, number of accelerated electrons, and magnetic field strength and direction can be found. In practice, obtaining complete radio images at a large number of frequencies is a significant technical challenge, especially while keeping costs down. Our instrument at Owens Valley Radio Observatory provided the starting point for a modest attempt at meeting this goal. We proposed to build three additional, very low-cost 2-m antennas which, when combined with our existing two 27-m dishes, expands the array to 5 elements. This modest increase in number of solar dedicated antennas, from 2 to 5, increases our maximum number of physical baselines from 1 to 10 and allows the instrument to do true imaging of solar microwave sources, both bursts and active regions. Combined with the technique of frequency synthesis, the new array has up to 450 effective baselines, giving imaging capability that approaches that of a sub-arrayed VLA. The prototype antenna design was finalized and the antenna was put into operation in Nov. 1989.

Hurford, Gordon↗
NASA NTRS1993

Newly found evidence of Sun-climate relationships

Solar radiation cycles drive climatic changes intercyclically. These interdecadal changes were detected as variations in solar total irradiances over the time period of recorded global surface-air-temperature (SAT) and have been restored utilizing Earth Radiation Budget Channel 10C measurements (1978-1990), Greenwich Observatory faculae data (1874-1975), and Taipei Observatory Active Region data (1964-1991). Analysis of the two separate events was carried out by treating each as a discrete time series determined by the length of each solar cycle. The results show that the global SAT responded closely to the input of solar cyclical activities, S, with a quantitative relation of T = 1.62 * S with a correlation coefficient of 0.61. This correlation peaks at 0.71 with a built-in time lag of 32 months in temperature response. Solar forcing in interannual time scale was also detected and the derived relationship of T = 0.17 * S with a correlation coefficient of 0.66 was observed. Our analysis shows derived climate sensitivities approximately fit the theoretical feedback slope, 4T(sup 3).

Kim, Hongsuk H.↗
NASA NTRS1993

Parameterized study of the ionospheric modification associated with sun-aligned polar cap arcs

The local ionospheric modification that is due to a generalized steady state solar aligned (SA) arc structure is addressed. For a representative set of SA arc parameters which includes both convection and precipitation, emphasis is placed on the modification by SA polar cap arcs upon the F region electron density and the height integrated conductivity. At low fluxes and low characteristic energies, SA polar cap arcs have the most pronounced relative effect at F region altitudes in darkness for winter solar minimum conditions. The absolute enhancement in summer solar minimum and winter solar maximum is equivalent to that of winter solar minimum, but the higher ambient densities make the relative enhancement less. The TEC enhancement associated with an SA arc may be used to indicate the degree of plasma cross flow across the arc.

Crain, D. J.↗
NASA NTRS1993

The correspondence between X-ray bright points and evolving magnetic features in the quiet sun

The results of a study of X-ray bright points (XBPs) and small-scale evolving magnetic structures are presented. X-ray images obtained during rocket flights, full-disk magnetograms, and time-lapse magnetograms of multiple fields make up the coordinated data set. XBPs were found to be more frequently associated with pre-existing magnetic features of opposite polarity which appeared to be cancelling than with new or emerging flux regions. Most of the XBPs appeared to correspond to opposite polarity magnetic features which were converging towards each other, and some of which had not yet begun cancelling. It is suggested that most XBPs are created when converging flow brings together oppositely directed field lines. This leads to reconnection and heating in the low corona of the newly-formed loops.

Webb, D. F.↗
NASA NTRS1993

An ultraviolet spectral atlas of the sun between 1190-1730 A

An ultraviolet spectral atlas of different solar regions in the wavelength region 1190-1730 A is presented. The data were obtained with the High Resolution Telescope and Spectrograph during a rocket flight in 1978. The spectral resolution of the instrument was 0.05 A while a spatial resolution of approximately 1.8 arcsec was achieved. The atlas contains spectra of three different quiet regions including pointings at disk center and the extreme limb. Data from an active region close to the solar limb and a prominence have also been included together with an atlas of a so-called explosive event. The selected areas are averaged over several arcsec, ranging from 3.5 to 220 arcsec. The absolute intensities are determined with an accuracy of +/- 30 percent except for the strongest emission lines that are synthesized from spectra with different exposure times. The atlas is available in a machine-readable form together with an IDL program to interactively measure line widths, total intensities, and solar wavelengths.

Brekke, P.↗
NASA NTRS1992

Cool stars, stellar systems, and the sun; Proceedings of the 7th Cambridge Workshop, Tucson, AZ, Oct. 9-12, 1991

Consideration is given to HST observations of late-type stars, molecular absorption in the UV spectrum of Alpha Ori, EUV emission from late-type stars, Rosat observations of the Pleiades cluster, a deep ROSAT observation of the Hyades cluster, optical spectroscopy detected by EXOSAT, stellar photospheric convection, a structure of the solar X-ray corona, magnetic surface images of the BY Dra Star HD 82558, a Zebra interpretatin of BY Dra stars, optical flares on II Peg, a low-resolution spectroscopic survey of post-T tauri candidates, millimeter and sub-millimeter emission from flare stars, and activity in tidally interacting binaries. Attention is also given to modeling stellar angular momentum evolution, extended 60-micron emission from nearby Mira variables, the PANDORA atmosphere program, the global properties of active regions, oscillations in a stratified atmosphere, lithium abundances in northern RS CVn binaries, a new catalog of cool dwarf stars, the Far UV Spectrograph Explorer, and development of reflecting coronagraphs.

Giampapa, Mark S.↗
NASA NTRS1992

Absorbed photosynthetically active radiation of steppe vegetation and sun-view geometry effects on APAR estimates

Instantaneous fractions of absorbed photosynthetically active radiation (APAR) were measured at the Streletskaya Steppe Reserve in conjunction with canopy bidirectional-reflected radiation measured at solar zenith angles ranging between 37 and 74 deg during the Kursk experiment (KUREX-91). APAR values were higher for KUREX-91 than those for the first ISLSCP field experiment (FIFE-89) and the amount of APAR of a canopy was a function of solar zenith angle, decreasing as solar zenith angle increased at the resrve. Differences in absorption are attributed to leaf area index (LAI) and leaf angle distribution and subsequently transmitted radiation interactions. LAIs were considerably higher at the reserve than those at the FIFE site. Leaf angle distributions of the reserve approach a uniform distribution while distributions at the FIFE site more closely approximate erectophile distributions. Reflected photosynthetically active radiation (PAR) components at KUREX-91 and FIFE-89 were similar in magnitude and in their response to solar zenith angle. Transmitted PAR increased with increasing solar zenith angle at KUREX-91 and decreased with increasing solar zenith angle at FIFE-89. Transmitted PAR at FIFE-89 was considerably larger than those at KUREX-91.

Walter-Shea, E. A.↗
NASA NTRS1993

Hotspots and sunspots - Surface tracers of deep mantle convection in the earth and sun

The evolution of the hot-spot distribution on earth in time and space is investigated using available age data. The statistics of continental flood basalt eruptions suggests the formation of a total of about 40 hot spots worldwide during the Cenozoic and Mesozoic, with no true antipodal pairs found. It was found that hot spots tend to concentrate mainly in mid-latitudes, but the pattern of new appearances of hot spots may migrate from high to low latitudes in both hemispheres in long cycles, and may also drift in longitude, although much more slowly prograde.

Stothers, Richard B.↗
NASA NTRS1993

Interactions among convection, magnetic fields and p-mode oscillations in the sun

Two papers on different aspects of the excitation and damping of solar oscillations were accepted for publication in the Astrophysical Journal. The first paper evaluates the rate at which turbulent convection feeds energy into individual p-modes. It is shown that stochastic excitation by turbulent convection provides a satisfactory fit to the product of the mode energies and linewidths. A somewhat surprising conclusion is that entropy fluctuations are about an order of magnitude more significant than are fluctuations of the Reynolds stress in exciting p-modes. However, entropy fluctuations cannot excite f-modes. This may account for the relatively low energies of the f-modes compared to those of the p-modes. The second paper explores the role of scattering of acoustic modes by turbulent velocity fluctuations. Scattering of a mode is concentrated near the top of its acoustic cavity. Because the turbulence has low Mach number, scattering couples modes having similar frequencies. Its net effects are to increase the linewidths of all modes and to transfer energy from p-modes to f-modes. Scattering is likely to be the dominant source for the linewidths of p-modes. In particular, it may account for the unexpectedly large linewidths measured for low frequency modes. Copies of preprints of the two papers referred to above are attached. The remainder of the report is devoted to a description of unpublished results.

Goldreich, Peter↗
NASA NTRS1988

Unmasking the Sun

This videotape describes solar-related research at the Mt. Palomar Observatory.

Source record↗
NASA NTRS1993

The young Sun and photochemistry of the primitive Martian atmosphere

We will investigate the climatology of an atmosphere where CO2 is a minor constituent but still the key radiative species. The thermal structure of the dust-free atmosphere is estimated by employing a simple radiative-convective model similar to that used by Gierasch and Goody. Radiative heating rates are computed using the Caltech/JPL one-dimensional photochemical model. Thermal cooling rates for a Martian atmosphere containing O2, O3, H2O, N2O, CO, and CO2 are calculated using FASCODE and k-distribution methods. The effects due to pressure broadening of the infrared absorption lines of CO2 by CO and O2, as well as the radiative effects of increased ozone densities in the atmosphere, will be examined.

Nair, H.↗
NASA NTRS1994

Analysis of data from the plasma composition experiment on the International Sun-Earth Explorer (ISEE 1)

The Lockheed plasma composition experiment on the ISEE 1 spacecraft has provided one of the largest and most varied sets of data on earth's energetic plasma environment, covering both the solar wind, well beyond the bow shock, and the near equatorial magnetosphere to a distance of almost 23 earth radii. This report is an overview of the last four years of data analysis and archiving. The archiving for NSSDC includes most data obtained during the initial 28-months of instrument operation, from early November 1977 through the end of February 1980. The data products are a combination of spectra (mass and energy angle) and velocity moments. A copy of the data user's guide and examples of the data products are attached as appendix A. The data analysis covers three major areas: solar wind ions upstream and downstream of the day side bowshock, especially He(++) ions; terrestrial ions flowing upward from the auroral regions, especially H(+), O(+), and He(+) ions; and ions of both solar and terrestrial origins in the tail plasma sheet and lobe regions. Copies of publications are attached.

Lennartsson, O. W.↗