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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 451 records · Page 25
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NASA NTRS1979

Stellar model chromospheres. IX - Chromospheric activity in dwarf stars

High-resolution Ca II K line profiles are used to model the upper photospheres and lower chromospheres of eight main-sequence stars ranging in spectral type from F0 to M0 and exhibiting different degrees of chromospheric activity. The model chromospheres are studied as a function of spectral type and activity for stars of similar spectral type in order to obtain evidence of enhanced nonradiative heating in the upper-photospheric models and in the ratio of minimum temperature at the base of the chromosphere to effective temperature, a correlation between activity and temperature in the lower chromospheres, and a correlation of the width at the base of the K-line emission core and at the K2 features with activity. Chromospheric radiative losses are estimated for the modelled stars and other previously analyzed main-sequence stars. The results obtained strengthen the argument that dMe flare stars exhibit fundamentally solar-type activity but on an increased scale.

Kelch, W. L.↗
NASA NTRS1979

Coronal holes and solar magnetic fields

Since 1972 nearly continuous observations of coronal holes and their associated photospheric magnetic fields have been made using a variety of satellite and ground-based equipment. The present paper reviews the results of comparisons of these data and shows that the structure and evolution of coronal holes is basically governed by the large-scale distribution of photospheric magnetic flux. Nonpolar holes form in the decaying remnants of bipolar magnetic regions in areas with a large-scale flux imbalance. In addition, there is strong indirect evidence that the magnetic field in coronal holes is always open to interplanetary space, but not all open-field regions have associated coronal holes.

Harvey, J. W.↗
NASA NTRS1979

The forecasting center of Meudon, France

Main features of solar activity are described in relation to solar and geophysical forecasting. Spectroheliograms, radio and X-ray data, white light coronal observations, particles data, photospheric images, and photospheric magnetic fields are among the types of data used to identify the active centers and flares of the Sun. Forecasting and identification of geomagnetic activity is also discussed. The forecasting technique is described along with the types of users.

Simon, P.↗
NASA NTRS1979

Elemental composition of solar energetic particles in 1977 and 1978

The elemental composition of energetic nuclei from seven major solar flare events were measured wit the cosmic ray detector systems aboard the Voyager 1 and 2 spacecraft. The energetic nuclei abundances differ significantly from those of photospheric material. They are enhanced relative to the photonsphere by a factor which is the ratio of abundance of an energetic nuclei species (relative to oxygen) over the corresponding abundance of photospheric material. This factor is common to all events and has a nonmonochromatic characteristic dependence on nuclear charge. This factor is roughly ordered by first ionization potential into two groups of elements, metallics and volatiles.

Cook, W. R.↗
NASA NTRS1980

The rate of mass loss and variations in the wind from the Be star delta Centauri

Copernicus ultraviolet scans of the Be star delta Centauri obtained in 1976 and 1979, show a significant variation in the Si III lambda 1206 profile, The strong asymmetry that was present in 1976 had disappeared by 1979. The Si IV lambda 1400 doublet was also asymmetric in 1976, but was not observed in 1979. A quantitative fit of the line shapes to theoretical wind profiles shows that the mass-loss rate in 1976 was 2 x 10 to the minus 8th power/yr, and that the rate of mass loss in Si III was at least one order of magnitude less in 1979. It is not possible to determine whether the variation represented an overall change in the lass-loss rate, or whether it was due to a change in the ionization balance. The profile fitting procedure resulted in the adoption of assumed underlying photospheric Si III and Si IV profiles, and the equivalent widths measured from these profiles are most consistent with T sub eff between 30,000 and 35,000 K, somewhat hotter than implied by the spectral classification normally assigned to this star. The ultraviolet photospheric line widths, coupled wit published theoretical analyses of rotational gravitational darkening, imply an intrinsic equatorial velocity of about 310 km/sec and an angle of inclination of the rotational axis to the line of sight of i is less than or equal to 44 deg.

Snow, T. P., Jr.↗
NASA NTRS1979

IUE observations of the X-ray source HZ Herculis/Her X-1

The results of a campaign of monitoring HZ Her with the IUE satellite are presented. The spectrum and light variation outside of eclipse are fairly consistent with the Milgrom and Salpeter (1975) model for emission from the X-ray heated photosphere of the primary. Near eclipse there is UV flux in excess of the prediction of the heated photosphere model. This suggests an additional emitting component whose relative importance decreases toward shorter wavelengths. The excess flux near 1500A appears to vary with 35 day phase in a manner consistent with the model of Gerend and Boynton (1976) in which the flux contributed by the accretion disk is modulated by precession.

Gursky, H.↗
NASA NTRS1980

HEAO 1 observations of gamma-ray lines from a solar flare

HEAO 1 observed gamma radiation, including the 2.223 MeV line of deuterium and the 4.43 MeV line of C-12, from a white-light solar flare of 1978 July 11. Line strengths over a 4 minute integration were 1.00 + or - 0.29 and 0.18 + or - 0.07 photons/sq cm s, respectively, and the continuum in the 1-5 MeV range fitted a spectrum 10 E to the -3rd photons/sq cm s keV. The 2.2 MeV line lagged 94 + or - 30 s behind the gamma-ray continuum, which itself was delayed about 20 s from the hard X-ray (not less than 40 keV) and microwave fluxes. This is the second flare for which both MeV-range lines and continuum have been observed, and the first for which simultaneous white-light observations exist. The prompt gamma-ray line (4.43 MeV) can be directly interpreted as an energy deposition of not greater than 7 x 10 to the 26th ergs per sec by energetic protons at photospheric depths. This is insufficient to maintain the white-light continuum by normal photospheric emission mechanisms.

Hudson, H. S.↗
NASA NTRS1980

Analysis of the solar magnesium lines

The observed quiet-sun profiles for a number of Mg I and II lines are compared with theoretical spectra computed for two upper-photosphere lower-chromosphere models published by Ayres and Linsky (1976). Both the Mg I and the Mg II resonance line wings, observed with the University of Hawaii Echelle Rocket Spectrographs, favor a model with a higher temperature in the upper photosphere than required to match the visible region lines 4571 and 5172. Neither model atmosphere reproduces the observed shapes in the cores of the strong lines or the limb darkening of the 4571 A intercombination line. The wings of the ultraviolet lines in plages can be reproduced either with a mean one-component atmosphere or by a two-component model with an ad hoc filling factor.

Heasley, J. N.↗
NASA NTRS1980

Depth-dependence of turbulence in stellar atmospheres

Recent observations concerning the depth dependence of motions in stellar atmospheres are reviewed. With regard to turbulence in stellar photospheres, attention is given to evidence from the sun and Arcturus for an increase in the total micro- plus macro-velocities with height, starting at mid-photospheric levels. Concerning stellar chromospheric velocity fields, it is noted that Mg II data point to changes in the hydrodynamic structure of the upper atmosphere. Ca II and Mg II asymmetry observations and other studies are used to describe the upper atmospheric structure of cool evolved stars, particularly coronae and circumstellar envelopes, revealing a depth dependent effect of velocity fields among a wide range of such stars.

Stencel, R. E.↗
NASA NTRS1980

Effects of granular convection in the response of C I 5380 A to solar luminosity variations

The response of the weak solar photospheric neutral carbon line at 5380.3 A to percolations of photospheric granulation and the five-minute solar oscillations is investigated in order to calibrate the sensitivity of the line to temporal variations in solar luminosity. Line strength variations and simultaneous continuum variations in two granular regions were observed at 30 sec intervals using a 25-cm coude spectrograph. The response of line equivalent width to continuum intensity variations is found to be essentially uniform at time scales ranging from 5 min to 2 hr independently of whether the variations arise from granular convection or velocity field oscillations. The extent to which line strength varies with luminosity (sensitivity) is observed to be significantly smaller than estimates based on models of the perturbed Harvard-Smithsonian Reference Atmosphere of Gingerich et al. (1970), and it is proposed that the structure of the solar granulation is a major factor determining local luminosity variations and line sensitivity.

Lindsey, C. A.↗
NASA NTRS1980

Origin of the warped heliospheric current sheet

The warped heliospheric current sheet for early 1976 is calculated from the observed photospheric magnetic field by a potential field method. Comparisons with measurements of the interplanetary magnetic field polarity for early 1976 obtained at several locations in the heliosphere by Helios 1, Helios 2, Pioneer 11, and at the earth show a rather detailed agreement between the computed current sheet and the observations. It appears that the large-scale structure of the warped heliospheric current sheet is determined by the structure of the photospheric magnetic field and that 'ballerina skirt' effects may add small scale ripples.

Wilcox, J. M.↗
NASA NTRS1980

Sunspots and the physics of magnetic flux tubes. X - On the hydrodynamic instability of buoyant fields

The hydrodynamics of cylindrical buoyant bubbles (magnetic flux tubes) in an atmosphere is examined. Their dynamical stability is considered, with particular attention to the Rayleigh-Taylor and Kelvin-Helmholtz instabilities. The instabilities are sufficient to cause fragmentation in the absence of some strong stabilizing effect such as surface tension or an internal azimuthal magnetic field. The results of the stability analysis are first applied to rising gas bubbles in liquids. Then, since agreement with the experimental data is found, the results are applied to the similar problem of the rising magnetic flux in the sun. Magnetic flux tubes in a fluid velocity field are intrinsically unstable, unless twisted. With the known dynamical limits on the degree of twisting, an upper limit on the diameter of the order of a few hundred kilometers is found for the stability of individual flux tubes in the photosphere. Observations show that tubes, except when clustered to form sunspots, are generally not larger than this amount. Therefore, it is suggested that the hydrodynamic instability of larger tubes is a prime cause of their absence from the solar photosphere.

Tsinganos, K. C.↗
NASA NTRS1980

The origin of the warped heliospheric current sheet

The warped heliospheric current sheet in early 1976 was calculated from the observed photospheric magnetic field using a potential field method. Comparisons with measurements of the interplanetary magnetic field polarity in early 1976 obtained at several locations in the heliosphere at Helios 1, Helios 2, Pioneer 11 and Earth show a rather detailed agreement between the computed current sheet and the observations. It appears that the large scale structure of the warped heliospheric current sheet is determined by the structure of the photospheric magnetic field, and that "ballerina skirt" effects may add small scale ripples.

Wilcox, J. M.↗
NASA NTRS1980

Elemental composition of solar energetic particles in 1977 and 1978

Measurements of the elemental composition of energetic nuclei with atomic numbers between 2 and 28 from seven major solar flares from September 1977 to May 1978 are presented. The abundance observations were made with the Low Energy Telescope systems of the cosmic ray detector systems on board the Voyager 1 and 2 spacecraft between 1 and 3 AU. Examination of the abundance ratios of the flare nuclei relative to oxygen reveals significant variations from event to event and between energetic nuclei and photospheric abundances, with an average composition, except for C and N, very similar to that of the galactic cosmic ray source. For the four flare events for which the elemental abundances exhibit no significant energy dependence in the energy range observed, it is found that the enhancement of energetic nuclei relative to their photospheric abundance are similar and not monotonic with atomic number, with the metallic nuclei showing an enhancement factor of approximately 5 and the volatiles showing one closer to 1.

Cook, W. R.↗
NASA NTRS1981

A comparison of Type III metric radio bursts and global solar potential field models

Evidence of coronal magnetic fields from polarized metric type III radio bursts is compared with (1) global potential field models, (2) direct averages of the observed photospheric magnetic field, and (3) H-alpha synoptic charts. The comparison clearly indicates both that the principal aspects of type III burst radiation are understood and that global potential field models are a significantly more accurate representation of coronal magnetic field structure than either the large-scale photospheric field or H-alpha synoptic charts.

Jackson, B. V.↗
NASA NTRS1981

Continuum emission in the 1980 July 1 solar flare

Comparison of continuum measurements of the July 1, 1980 flare at Big Bear Solar Observatory and Sacramento Peak Observatory show strong blue emission kernels with the ratio of Balmer continuum (Bac):3862 A continuum:continuum above 4275 A to be about 10:5:1. The blue continuum at 3862 A is too strong to be explained by unresolved lines. The Bac intensity was 2.5 times the photosphere and the strongest 3826 A continuum was 2 times the photosphere. The brightest continuum kernel occurred late in the flare, after the hard X-ray peak and related in time to an isolated peak in the 2.2 MeV line, suggesting that that continuum was excited by protons above 20 MeV.

Zirin, H.↗
NASA NTRS1981

Solar abundances from X-ray flare observations

X-ray line and continuum observations are used to derive the absolute coronal abundances of Si, S, Ar, and Ca from lines of hydrogenor helium-like ions. For Si (7.7+0.2, -0.3) and Ca (6.5+0.1, -0.3) the measured coronal values are in good agreement with the photospheric values, but the value for S (6.9+0.1, -0.3) is approximately half of the photospheric value. The abundance of Ar (6.4+0.2, -0.3) also appears to be lower in the corona by a factor of 2-3 than would be expected. It is suggested that these differences are due to ambipolar and magnetic diffusion effects.

Veck, N. J.↗
NASA NTRS1981

Short and long term variations in the solar constant

Short and long term variations in the solar constant are examined theoretically. The variations observed by the Solar Maximum Mission, lasting several days and associated with the passage of sunspot groups, strikingly demonstrates the well known lack of a bright ring effect around sunspots. This suggests that sunspot magnetic fields do not simply block the heat flowing upward into the photosphere. Rather, it is suggested that gravitational draining occurs; this cools sunspots and transports downward the heat that would otherwise flow into the photosphere. A model of sunspot temperature with depth shows modest support when compared with the empirical model of Van't Veer. Secular trends in the solar constant may occur and be associated with the influence of the convection zone magnetic field upon convective heat transport. As a start to understanding this problem, the Schwarzschild criterion has been modified to include the effects of magnetic field.

Schatten, K. H.↗