Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “PHOTOSPHERE”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

The stability of solar coronal loops with realistic photospheric boundary conditions

Finite-length effects are the primary influence for the MHD stability of coronal loops. This paper gives a new physical description of the photospheric boundary conditions, and the first complete and uniformly convergent initial-perturbation set. A general energy-principle (or variational) stability analysis is then developed. The results of this calculation show that large classes of cylindrical-symmetry perturbations, those with separable contributions to the energy integral, are completely stable. Comparisons are also made with other more restrictive formulations of this problem.

Van Hoven, G.↗
NASA NTRS1981

Active regions from the photosphere to the chromosphere

The structures and physical conditions in that portion of a solar active region extending from the photosphere to the corona are considered. A horizontally averaged model of a solar plage is developed which is in reasonable agreement with observations, and significant discrepancies existing between quiet sun models and UV and IR spectra are noted which cast doubt on the reliability of such models. The nature of the flux tubes comprising the small-scale structure of the active regions is discussed, and the concept of filling factor is described as a basis for a flux tube model explaining changes in spectral properties. The effects of two-dimensional radiative transfer are examined for flux tubes of various cross-sectional size, and it is found that the neglect of explicit horizontal radiative interactions is a good approximation to the two-dimensional treatment. The radiative power loss from active regions is discussed for homogeneous and flux tube models, and a significant difference in total heating requirements is found. Finally, attention is given to the characteristics of the transition zone.

Chapman, G. A.↗
NASA NTRS1982

The equatorial photospheric rotation rate

The equatorial photospheric rotation rate was observed on 14 days in 1978-1980. The resulting rate, 14.14 + or - 0.04 deg/day, is found to be 2% slower than the rate observed for long-lived sunspots. Tables giving sidereal equatorial rotation rates measured with the line 10905 A Cr I and 6315 A Ni I are included.

Duvall, T. L., Jr.↗
NASA NTRS1981

The spontaneous concentration of magnetic field in the photosphere of the sun

The basic physics of magnetic flux tubes in the solar photosphere is reviewed, with areas still open to conjecture pointed out. The question of the concentration of individual small flux tubes to levels of 1-2 kilogauss, when the average solar surface magnetic field is on the order of 10 gauss, by processes of twisting and the formation of flux ropes made up of tubes wound around each other is considered together with the effects of turbulence on the flux tube. Mechanisms for tube compression by the evacuation of the gas contained within a flux tube are then examined, and the possibility of field concentration through the cooling of the gas within the tube in a superadiabatic process is suggested. Attention is then given to possible mechanisms serving to maintain the concentration of flux tubes far below the surface of the sun which gives rise to sunspots and pores as the flux tree emerges through the surface.

Parker, E. N.↗
NASA NTRS1982

Vector magnetic field evolution, energy storage, and associated photospheric velocity shear within a flare-productive active region

Sheared photospheric velocity fields inferred from spot motions for April 5-7, 1980, are compared with both transverse magnetic field orientation changes and with the region's flare history. Rapid spot motions and high inferred velocity shear coincide with increased field alignment along the longitudinal neutral line and with increased flare activity, while a later decrease in velocity shear precedes a more relaxed magnetic configuration and decrease in flare activity. It is estimated that magnetic reconfiguration produced by the relative velocities of the spots could cause storage of about 10 to the 32nd erg/day, while flares occurring during this time expended no more than about 10 to the 31st erg/day.

Krall, K. R.↗
NASA NTRS1982

The photospheric velocity field of Procyon

Balloon-borne Ultraviolet Stellar Spectrometer observations of the profiles of two well observed spectral lines in the ultraviolet spectrum of Alpha CMI (Procyon; F5IV-V) are analyzed with a Fourier transform method in order to determine values of various parameters of the velocity field of the upper photosphere. A microturbulent line-of-sight velocity component of 0.9 + or - 0.4 km/s, a macroturbulent velocity component of 5.3 + or - 0.2 km/s, and a rotational velocity component of 10.0 + or - 1.2 km/s were determined. In these calculations a single-moded sinusoidal isotropic macroturbulent velocity function was assumed. The result appears to be sensitive to the assumed shape of the macroturbulence function.

De Jager, C.↗
NASA NTRS1982

Hydromagnetic stability of coronal arcade structures The effects of photospheric line tying

A model is given of the magnetic-field equilibrium and possible dynamic excitations of a solar coronal arcade. Such structures are well observed in the spectral range from H-alpha to X-rays and often give rise to two-ribbon flares. However, the preflare state must be stable to ideal magnetohydrodynamic disturbances, and this problem is treated with particular attention to the necessary foot-point boundary conditions. With reasonably general perturbation set, an energy-principle analysis is used to show the strong stabilizing influence of inertial field-line tying at the photosphere.

Ray, A.↗
NASA NTRS1982

Hot mantles, moderate photospheres for Wolf-Rayet stars

The amount of continuous energy from Wolf-Rayet stars and the shape of the continuous spectrum from the ultraviolet to the near infrared correspond to effective temperatures in the range 25,000 to 30,000 K. The value of log g is of the order of 4.0 + or - 0.5. Thus the photospheres of Wolf-Rayet stars correspond to those of moderately hot stars. The line spectra of Wolf-Rayet stars, however, indicate that electron temperatures greater than 30,000 K occur in the outer atmospheres or mantles of these stars. Here outflow is important.

Underhill, A. B.↗
NASA NTRS1982

The detection of ultraviolet photospheric absorption in the spectra of two Wolf-Rayet stars

Narrow, unshifted UV absorption lines, principally of Fe v, have been detected in the spectra of two Wolf-Rayet stars, HD 93162 (WN7) and HD 193793 (WC7). Both stars are known to show upper Balmer-line absorption but exhibit no periodic Doppler shifts that might indicate the presence of a companion. If intrinsic to the Wolf-Rayet stars, these lines presumably form near the base of the envelope in the photosphere. The potential significance of absorption-line studies of Wolf-Rayet stars is discussed.

Fitzpatrick, E. L.↗
NASA NTRS1983

On possible correlations in the photospheric magnetic field

A correlation analysis is performed upon the rate of change of the photospheric magnetic field in the north and south hemispheres of the sun. Both active region logs and data obtained from the daily Mount Wilson magnetograms are employed, and the analysis is performed for all available days, as well as for days in which active regions are near central meridian. The result of the analysis shows no correlation between these changes and it is concluded that local convective turbulence is more important than dynamo processes with regard to the appearance of individual active regions.

Wallenhorst, S. G.↗
NASA NTRS1983

Magnetic rotation of the solar photosphere

Magnetograms made at Mt. Wilson Observatory from January 1967 to May 1982 are crosscorrelated in 34 latitude strips at 1-4-day increments to determine the rotation of magnetic features in the solar photosphere. The data are smoothed by averaging corresponding correlations and calculating rotation from the displacement of the averaged-correlation maximum; the usefulness and validity of this procedure are discussed. No significant time variation or field dependence is found for the period of the observations, at least to the accuracy of the calculated means (variance of from about 2 m/sec at low latitudes to about 10 m/sec near the poles). The rotation function omega at solar latitude phi is shown to be 2.902 0.464 sin sq phi - 0.328 sin to the 4th phi microrad/sec, in agreement with the Mt. Wilson Doppler profile near the poles and with the sunspot determination of Newton and Nunn (1951) at sunspot latitudes, where the Doppler estimate is about 30 m/sec slower.

Snodgrass, H. B.↗
NASA NTRS1984

SMM observations of K-alpha radiation from fluorescence of photospheric iron by solar flare X-rays

High-resolution Fe K-alpha spectra near 1.94 A observed during solar flares with the Bent Crystal Spectrometer on the Solar Maximum Mission are presented. The evidence for two possible excitation mechanisms, electron impact and fluorescence, is examined. It is found that the fluorescence mechanism satisfactorily describes the results, while the observations do not support electron collisional excitation of the Fe K-alpha transitions in low ionization stages (II-XII) of iron. Using Bai's model of the fluorescent excitation process, the photospheric iron abundance relative to that of hydrogen is estimated to be 5-6 x 10 to the -5th. The mean height of the soft X-ray source producing the K-alpha fluorescence is calculated on the basis of this model for about 40 large flares. The solar K-alpha lines are found to be about 25 percent wider than those measured in the laboratory. Weak line features observed at wavelengths shorter than that of the K-alpha lines are discussed.

Parmar, A. N.↗
NASA NTRS1985

Contribution functions for Zeeman-split lines, and line formation in photospheric faculae

The transfer of polarized light in an inhomogeneous stellar atmosphere, and the formation of magnetically sensitive spectral lines, are discussed. A new method for the solution of the transfer equations is proposed. The method gives a natural definition of the contribution functions for Stokes' parameters, i.e., functions describing the contributions from different parts along the line-of-sight (LOS). The formalism includes all magneto-optical effects, and allows for an arbitrary variation of magnetic field, velocity field, temperature, density, etc., along the LOS. The formation of FeI lambda 5250.2 in photospheric faculae is described. A potential-field model of a facular element is presented, and spectra profiles and contribution functions are computed for the Stokes parameters I, Q, and V.

Vanballegooijen, A. A.↗
NASA NTRS1984

Separation of large-scale photospheric Doppler patterns

Mount Wilson solar Doppler data covering the period from January 1967 to March 1984 are refit using polynomials appropriate to the rotation, limbshift, and meridional flow motion fields identified by LaBonte and Howard (1982). No 'ear' terms are included, and additional degrees of freedom are allowed in the limbshift and meridional flow fluctions. An examination of the residuals shows this fit to be a slight improvement over that of the above workers, and the new residuals are used to tighten the limits on photospheric giant signatures to about 1 m/s per wavenumber. The focus of the paper is on polynomial functions and their fit-determined coefficients, with particular emphasis on the problem of crosstalk as the coefficients are tracked over time.

Snodgrass, H. B.↗
NASA NTRS1986

A comparison of photospheric electric current and ultraviolet and X-ray emission in a solar active region

This paper presents an extensive set of coordinated observations of a solar active region, taking into account spectroheliograms obtained with the aid of the Solar Maximum Mission (SMM) Ultraviolet Spectrometer Polarimeter (UVSP) instrument, SMM soft X-ray polychromator (XRP) raster maps, and high spatial resolution ultraviolet images of the sun in Lyman-alpha and in the 1600 A continuum. These data span together the upper solar atmosphere from the temperature minimum to the corona. The data are compared to maps of the inferred photospheric electric current derived from the Marshall Space Flight Center (MSFC) vector magnetograph observations. Some empirical correlation is found between regions of inferred electric current density and the brightest features in the ultraviolet continuum and to a lesser extent those seen in Lyman-alpha within an active region.

Haisch, B. M.↗
NASA NTRS1986

Spectral flux from low-density photospheres - Approximate results

Radiative transfer through sharp, quasi-static atmospheres containing mostly hydrogen is considered at densities low enough that scattering dominates absorption and collisional excitations are negligible. It is found that the balance between photoionization and radiative recombination which governs the atomic level populations greatly reduces the effective absorptive opacity. Approximate results for the emergent continuum spectral flux are obtained, significantly different than if local thermodynamic equilibrium level populations were assumed. The flux jumps at the photoionization thresholds are relatively small for the photospheric temperatures T sub p(v) greater than or about 6000 K considered, and T sub p increases toward the far-ultraviolet. It appears that the lack of a Balmer jump in spectra of Type II supernovae implies that the flux is diluted below that of a blackbody at the same color temperature, which increases the value of the Hubble constant obtained by the Baade (1926) method.

Hershkowitz, S.↗
NASA NTRS1985

The cyclical variation of energy flux and photospheric magnetic field strength from coronal holes

The average soft X-ray emission from coronal holes observed on images obtained during rocket flights from 1974 to 1981 is measured. The variation of this emission over the solar cycle was then compared with photospheric magnetic flux measurements within coronal holes over the same period. It was found that coronal hole soft X-ray emission could be detected and that this emission appeared to increase with the rise of the sunspot cycle from activity minimum to maximum. These quantitative results confirmed previous suggestions that the coronal brightness contrast between holes and large-scale structure decreased during this period of the cycle. Gas pressures at the hole base were estimated for assumed temperatures and found to vary from about 0.03 dyne/sq cm in 1974 to 0.35 dyne/sq cm in 1981. The increase in coronal hole X-ray emission was accompanied by a similar trend in the surface magnetic flux of near-equatorial holes between 1975 and 1980 (Harvey et al., 1982).

Webb, D. F.↗