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At least 1,117 records · Page 62
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NASA NTRS1982

International ultraviolet explorer observations of Hyades stars

A description is presented of International Ultraviolet Explorer (IUE) satellite observations of transition region and chromospheric emission from a group of Hyades dwarfs which are strong X-ray emitters as seen in a survey conducted by Stern et al. (1981). Short-wavelength spectra (1175-2000 A) and long-wavelength spectra (1900-3200 A) have been obtained. Although the IUE sensitivity limit did not make it possible to detect emission lines in three stars, the presence of chromospheres and transition regions could be confirmed in BD +15 deg 640, 70 Tau, BD +14 deg 693, and BD +16 deg 592. The differential emission measure has been plotted as a function of temperature for the four considered stars.

Zolcinski, M.-C. S.↗
NASA NTRS1982

Vertical flows of supergranular and mesogranular scale observed on the sun with OSO 8

A program of observations was carried out in order to study the penetration of supergranular flows over a broad range of heights in the solar atmosphere. Steady Doppler velocities are determined from observations of a Si II spectral line using the Ultraviolet Spectrometer on the Orbiting Solar Observatory 8 (OSO 8) satellite and Fe I and Mg I lines with the diode-array instrument on the vacuum telescope at Sacramento Peak Observatory (SPO). The heights of formation of these spectral lines span about 1400 km or nearly 11 density scale heights from the photosphere to the middle chromosphere. Steady vertical flows on spatial scales typical of supergranulation and mesogranulation have been detected in the middle chromosphere with OSO 8. The patterns of intensity and steady velocity of granular scale are reproducible in successive data sets. The patterns appear to evolve slowly over the 9 hr period spanned by six orbits.

November, L. J.↗
NASA NTRS1982

Spatial and temporal evolution of soft and hard X-ray emission in a solar flare

Hard X-ray burst spectrometer and imaging spectrometer data are used to study the spatial and temporal characteristics of the 3.5-30.0 keV emission in an Apr. 10, 1980 solar flare. It is found that: (1) continuous energy release is needed to sustain the increase of the emission through the flare's rising phase, before and after the impulsive phase in hard X-rays, and the release is characterized by the production of 50 million-150 million K thermal regions within the flare loop structures; (2) the observational parameters which characterize the impulsive burst indicate that it is probably associated with nonthermal processes, such as particle acceleration; and (3) the continuous energy release is associated with strong chromospheric evaporation, in view of spectral line behavior. Both particle acceleration and chromospheric evaporation stop just before flare maximum, and the subsequent evolution is probably governed by the radiative cooling of the flare plasma.

Machado, M. E.↗
NASA NTRS1982

Outer atmospheres of cool stars. XII - A survey of IUE ultraviolet emission line spectra of cool dwarf stars

Quantitative information is obtained on the chromospheres and transition regions of M dwarf stars, in order to determine how the outer atmospheres of dMe stars differ from dM stars and how they compare with the outer atmospheres of quiet and active G and K type dwarfs. IUE spectra of six dMe and four dM stars, together with ground-based photometry and spectroscopy of the Balmer and Ca II H and K lines, show no evidence of flares. It is concluded, regarding the quiescent behavior of these stars, that emission-line spectra resemble that of the sun and contain emission lines formed in regions with 4000-20,000 K temperatures that are presumably analogous to the solar chromosphere, as well as regions with temperatures of 20,000-200,000 K that are presumably analogous to the solar transition region. Emission-line surface fluxes are proportional to the emission measure over the range of temperatures at which the lines are formed.

Linsky, J. L.↗
NASA NTRS1982

High resolution EUV spectroscopy of 56 Pegasi (KO 2p + wd)

The hypothesis that X-ray and UV emission by 56 Pegasi (K0II p + wd) originates in a conventional chromosphere, transition region and corona, as opposed to Schindler's hypothesis of accretion of the cool stellar wind onto the white dwarf companion, is discussed. Analysis of IUE data indicates that within the uncertainties associated with determining the stellar radius and abundances, the chromosphere, transition region, and corona of 56 Peg are similar to that of Beta Draconis both in structure and energy balance. The accretion hypothesis is not a unique explanation of the observed spectral properties and the white dwarf companion may not play a direct role. Furthermore, 56 Peg, according to current values of V-R and luminosity class, is on the nonsolar side of the Linsky-Haisch dividing line whereas Beta Draconis is on the solar side.

Jordan, C.↗
NASA NTRS1981

Dynamic phenomena in coronal flux tubes

The study of stellar atmospheres and the determination of specific physical mechanisms, geometries, and magnetic structures by which coronae are maintained is examined. Ultraviolet and soft X-ray components observed in the radiative output of cool stars and the Sun require counterentropic temperature gradients for their explanation. The existence of a hot corona is recognized as a result of mechanical or fluid dynamic effects and the importance of the magnetic field in the heating is accepted. Magnetohydrodynamic energy release associated with the emergence of magnetic flux through the chromosphere and its dynamic readjustment in the corona are major counterentropic phenomena which are considered as primary candidates for corona heating. Systematic plows in coronal flux tubes result from asymmetric heating and systematic flows can exist without substantial chromospheric pressure differences.

Mariska, J. T.↗
NASA NTRS1981

The generation of acoustic energy from stellar convection zones

The heating of stellar chromospheres and coronae by the dissipation of acoustic waves is an important heating mechanism. It is only in the lower solar chromosphere that short period acoustic wave heating seems undisputed. The arguments leading to the rejection of the so called acoustic heating theory are derived from comparisons of calculated acoustic energy fluxes with observational or theoretical requirements.

Bohn, H. U.↗
NASA NTRS1982

A three-component model of the variability of the solar ultraviolet flux 145-200 nM

The variability of the ultraviolet flux between 145 and 200 nm over both the eleven-year cycle and the 27-day solar rotation period is examined in terms of chromospheric activity, as determined from ground-based observations of the CaII K chromosphere. A three-component model of the solar UV flux is developed which includes the contributions to the full disk flux from both plage and active network emission. Solar cycle and solar rotation variations derived from the model are compared with the results of satellite and rocket experiments and with the two-component model of Cook et al (1980). Finally, possible ways of improving the model are discussed.

Lean, J. L.↗
NASA NTRS1982

Active-region evolution and solar rotation variations in solar UV irradiance, total solar irradiance, and soft X rays

Variations in the total solar irradiance, solar UV spectral irradiance, and solar soft X-ray emission caused by active region evolution and solar rotation are analyzed by using concurrent measurements from the NIMBUS 7 and GOES satellites. The observations are interpreted by using simple empirical models that relate ground-based observations of the size and location of sunspots and plages to the full-disk temporal variations. It is found that the major dips in the photospheric total solar irradiance S, which are evident in both satellite measurements and model predictions, are usually not accompanied by outstanding enhancements in the chromospheric and upper photospheric UV spectral irradiance or coronal X rays. The main cause of this difference between the variability of S and of the UV flux is that the total chromospheric plage enhancements are not outstanding at those times when the total sunspot are outstanding. X rays are even more variable because of a much wider CMD sensitivity.

Donnelly, R. F.↗
NASA NTRS1983

Numerical simulations of loops heated to solar flare temperatures. I - Gasdynamics. II - X-ray and UV spectroscopy

The NRL's Dynamic Flux Tube Model is used to numerically simulate the dynamic response of a coronal magnetic loop to an energy input of the order encountered in solar flares. The coronal plasma is heated by the deposition of flare energy at the top of the loop to more than 10 million K, yielding a conduction front that moves toward the chromosphere, where the plasma is heated by the large downward conductive flux and ablates upward to the coronal part of the loop at velocities of a few hundred km/sec. The conduction front simultaneously produces chromospheric ablation and compresses the material ahead of it. With the aid of compressional instabilities, the compressed plasma grows throughout the flare heating phase, presenting a possible source of the flare optical continuum emission which is correlated with soft X-ray radiation. The observational consequences of rapidly heated loop gas dynamic processes are discussed. In the second part of this presentation, the dynamical calculation results previously obtained are used to predict the spectral line intensities, profiles and wavelengths of several X-ray lines and the UV line of Fe XXI at 1354.1 A. Three different viewing orientations of the loop are considered.

Cheng, C.-C.↗
NASA NTRS1982

Energy balance and stability

The energy balance of the outer atmospheres of solarlike stars is discussed. The energy balance of open coronal regions is considered, discussing the construction and characteristics of models of such regions in some detail. In particular, the temperature as a function of height is considered, as are the damping length dependence of the global energy balance in the region between the base of the transition region and the critical point, and the effects of changing the amount of coronal heating, the stellar mass, and the stellar radius. Models of coronal loops are more briefly discussed. The chromosphere is then included in the discussion of the energy balance, and the connection between global energy balance and global thermal stability is addressed. The observed positive correlations between the chromospheric and coronal energy losses and the pressure of the transition region is qualitatively explained.

Hammer, R.↗
NASA NTRS1982

Serf studies of mass motions arising in flares

It is believed that radio type IVs, co-spatial with dense hot plasmoids, may be the result of a plasma radiation emission mechanism. The injection of mass into the corona was recently observed in chromospheric and coronal lines with magnetic field changes and also at very high speeds into loops. The start time of coronal loop transients, if extrapolated to the chromosphere, in most cases precedes flare H-alpha or X-ray emission. Observational inferences from polarization and other studies are seen as favoring the three-dimensional bubble over the planar loop as a description of coronal mass motions.

Wagner, W. J.↗
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

Einstein and IUE observations of nearby red dwarfs

This paper summarizes data for 40 representative nearby stars from one guest observer's coordinated programs with the Einstein Observatory or IUE Observatory. The coronal X-ray, chromospheric ultraviolet, and auxiliary optical properties of sets of these stars are tabulated or illustrated in several ways. Factors of stellar duplicity are shown to be quite prevalent in presenting the observations. The most luminous X-ray dwarfs below the sun are strongly prone to binary status. X-ray luminosity, and the ratio of chromospheric flux to X-ray flux, are dependent on photospheric radius. A very long period BY Draconis variable of type dM6e (HH And) is a detected X-ray source, and some presumably quite old (halo) stars are detected.

Johnson, H. M.↗
NASA NTRS1984

Observations of the brightness profile of the sun in the 30-200 micron continuum

The brightness profile of the quiet sun in broad continuum passbands centered at 30, 50, 100, and 200 microns was observed with a resolution of 2 arcmin. Weak radial darkening was seen at all four wavelengths near disk center. This reverses to brightening toward the limb in the 100 and 200 micron continuum. Radial darkening at 100 and 200 microns is not expected from smooth model chromospheres consistent with absolute brightness measurements. These results do not support a homogeneous model of the low chromosphere, where the temperature reversal occurs.

Lindsey, C.↗
NASA NTRS1984

Dynamics of solar filaments. III - Analysis of steady flows in H-alpha and C IV lines

A statistical analysis of intensities and radial velocities of several solar plage filaments (i.e. prominences seen on the solar disk) observed at disk center is presented. Intensity and radial (= vertical) velocity maps were derived from simultaneous 2D measurements of the H-alpha chromospheric line with the Multichannel Subtractive Double Pass spectrograph operating on the Meudon solar tower, and also obtained from 2D recordings of the C IV transition-zone line (1548 A) with the UV Spectrometer and Polarimeter aboard the Solar Maximum Mission satellite. A good correlation (around 0.5) is found between intensities in both lines, as well as between velocities. Persistent upflows are measured in both lines at the filament location. The mean vertical velocities are respectively 0.5 km/s in H-alpha and 5.6 km/s in C IV. The analysis of mass fluxes suggests that C IV upflows occur in the transition region around prominences rather than below, in the chromosphere-corona transition zone.

Schmieder, B.↗
NASA NTRS1985

Evidence for Non-radiative Activity in Stars with T Sub Eff 10000 K

Major advances in the acquisition of evidence for and the understanding of nonradiative heating and other activity in stars cooler than T sub eff = 10,000 K has occurred in the last few years primarily as a result of the IUE and Einstein spececraft. This evidence is critically reviewed and trends that are now becoming apparent are noted. The existence for nonradiatively heated outer atmospheric layers (chromospheres, transition regions, and coronae) in dwarf stars cooler than spectral type A7, in F ang G giants, pre-main sequence stars, and close binary systems in unambiguous, and chromospheres exist in the K and M giants and supergiants. The existence of nonradiative heating in the outer layers of the A stars remains undetermined despite repeated searches at all wavelengths. Some important trends in these data are the decrease in plasma emission measure with age on the main sequence and decreasing rotational velocity. Variability and atmospheric inhomogeneity is commonly seen, and there is considerable evidence that magnetic fields define the geometry and control the energy balance in the outer atmospheric layers.

Linsky, J. L.↗
NASA NTRS1985

Thermal stability of static coronal loops: Part 1: Effects of boundary conditions

The linear stability of static coronal-loop models undergoing thermal perturbations was investigated. The effect of conditions at the loop base on the stability properties of the models was considered in detail. The question of appropriate boundary conditions at the loop base was considered and it was concluded that the most physical assumptions are that the temperature and density (or pressure) perturbations vanish there. However, if the base is taken to be sufficiently deep in the chromosphere, either several chromospheric scale heights or several coronal loop lengths in depth, then the effect of the boundary conditions on loop stability becomes negligible so that all physically acceptable conditions are equally appropriate. For example, one could as well assume that the velocity vanishes at the base. The growth rates and eigenmodes of static models in which gravity is neglected and in which the coronal heating is a relatively simple function, either constant per-unit mass or per-unit volume were calculated. It was found that all such models are unstable with a growth rate of the order of the coronal cooling time. The physical implications of these results for the solar corona and transition region are discussed.

Antiochos, S. K.↗