The solar O i λ 7773 triplet

Profiles of the O i 7773 triplet obtained at a spatial resolution 0.5 are analyzed using spectral line inversion techniques. Inferences are made about departures from LTE, convective velocity fields, and solar temperature fluctuations.     

The solar Lα flux near solar minimum

After being turned off in 1972 the OSO-5 satellite was reactivated during the summer of 1974 for one year. The University of Paris experiment designed to monitor the solar L flux operated almost perfectly during that period which occurred near a minimum in solar activity. This new set of data is presented here, showing that neither the total L flux nor the flux emitted at the center of the line correlate with the solar activity indices in the same manner as previously found at higher levels of activity.These new observations confirm that the solar L flux varies approximately by a factor of two from solar maximum to solar minimum.Furthermore, the lower boundary of the transition region seems to be strongly perturbated near solar minimum, since the flux variations observed at the center of the L line are drastically different from all those previously reported. This seems to be related to the presence of large coronal holes over the Sun.     

The Group Of λ Bootis Stars

In this article the current status of the knowledge forthe group of λ Bootis stars is reviewed. These metal poor objectsare quite outstanding compared to other chemically peculiar stars of theupper main sequence. Up to now no conclusive theory has been developedwhich is able to explain the majority of observational results. There is,however, an increasing interest in observational as well as theoreticalresults in order to develop and test astrophysical models such asaccretion of circumstellar matter and diffusion processes in stellaratmospheres. This revised version was published online in July 2006 with corrections to the Cover Date.     

X-ray bright points and He i λ 10830 dark points

Using near-simultaneous full disk solar X-ray images and Hei 10830 spectroheliograms from three rocket flights, we compare dark points identified on the Hei maps with X-ray bright points identified on the X-ray images. We find that for the largest and most obvious features there is a strong correlation: most Hei dark points correspond to X-ray bright points. However, about two-thirds of the X-ray bright points were not identified on the basis of the helium data alone. Once an X-ray feature is identified it is almost always possible to find an underlying dark patch of enhanced Hei absorption which, however, would not a priori have been selected as a dark point. Therefore, the Hei dark points, using current selection criteria, cannot be used as a one-to-one proxy for the X-ray data. Hei dark points do, however, identify the locations of the stronger X-ray bright points.Visitor, National Solar Observatory. National Optical Astronomy Observatories operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

Convective velocities derived from granule contrast profiles in Fe i λ 6569.2

A new determination of the granular and intergranular velocities is described, based on a new approach. The method involves measurement of the granule/intergranule contrast as a function of wavelength on a sequence of filtergrams taken with the CSIRO computer-controlled 1/8 Å filter in the photospheric line Fe i 6569.2. A procedure based on a simple but realistic morphological model of the granulation pattern is used to correct for spatial smearing. The effects of spectral smearing and of scattered light are also taken into account.The present observations reveal a one hundred per cent correlation between brightness and the sense of the vertical velocity component and thus demonstrate beyond doubt the convective origin of the granulation. The new measurements yield a value of 1.8 km s^–1 for the difference between the upward and downward velocities associated with an average granule. With certain plausible assumptions this leads to granular and intergranular velocities of 0.7 km s^–1 (upward) and 1.1 km s^–1 (downward) respectively.Estimates are also obtained for the (true) central intensities and line broadening parameters of the line profile, separately for the average granule and intergranular lane.     

Effect of small-scale magnetic fields on magnetically sensitive Fe I λ 525.02 and λ 1564.85 nm line profiles in the quiet solar photosphere

An integrated effect of small-scale magnetic fields on online absorption coefficient is analytically estimated. The formation of magnetically sensitive Fe I lines under the conditions of undisturbed solar photosphere in the presence of small-scale magnetic fields is studied. It is shown that these fields can broaden the wings of magnetically sensitive lines.     

NLTE formation of the resonance Ba II line λ 455.4 nm in the solar atmosphere

We consider the NLTE formation of the resonance Ba II line λ 455.4 nm in the solar spectrum for three one-dimensional and one three-dimensional hydrodynamic models of the quiet solar atmosphere. The sensitivity of the line to atomic parameters, microturbulent and macroturbulent velocities, as well as to oscillator strength and barium abundance uncertainties was examined. The wings of the barium line are shown to be most sensitive to the van der Waals broadening constant. Another important parameter is the barium abundance. Our NLTE estimate of the solar barium abundance (A _Ba = 2.16) derived with allowance made for the nonuniform solar atmosphere structure is in good agreement with earlier results. The influence of granular convective motions on the line profile shape was studied, and the profiles formed in granules and in intergranular lanes are shown to be asymmetric and differently shaped. We demonstrate that the theoretical profiles match well the observed ones when the NLTE effects and the granular structure are taken into account.     

The solar radiation between 3300 and 12500 Å

Results are presented, which follow from the merging of: (a) our previously published absolute integrals of the disk-center intensity for 20 Å wide spectral bands; (b) the ratios of mean to central intensity derived from recent observations of the center-to-limb variation of those bands ( <6600 Å); (c) the ratios of mean to central intensity derived from the observations of the center-to-limb variation at continuum-wavelengths according to Pierce and Slaughter ( >6600 Å); (d) the high resolution Fourier transform spectra obtained by J. Brault at Kitt Peak for the disk-center and the irradiance; (e) some further auxiliary data, which served mainly to eliminate the local perturbations caused by lines of telluric molecular bands.The main result is the presentation of high precision radiation data for both the integrated disk and the disk-center, concerning the line-averaged radiation and the continuum (in UV: highest window-intensities) as well.The internal accuracy (the scatter) should not be worse than that of the FTS spectra, which is less than 0.2% (mean error); local systematic deviations exceeding 0.5% are not to be expected. The absence of a significant systematic error - neutral or wavelength-dependent - has been proven already elsewhere.     

Diagnostics of horizontal velocity field in the solar atmosphere: Line Ba II λ 455.403 nm

We proposed a method for diagnostics of the horizontal velocity field based on 2D observations at the center of the solar disk with high spatial and temporal resolution. The method consists in semiempirical modeling of the solar atmosphere by solving the inverse radiative transfer problem and subsequent obtaining horizontal velocities by solution of the corresponding hydrodynamic equations. We investigated the diagnostic capabilities of the line Ba II λ 455.403 nm (considering hyperfine structure and isotope splitting) for studying the horizontal velocity field of the nonhomogeneous solar atmosphere.     

The solar power spectrum — a fractal set?

A preliminary numerical analysis of the power spectrum of solar oscillations of the SCLERA group suggests that this curve can be characterized by a Hausdorff-Besicovich dimension close to 3/2 near the present observational resolution (0.03 mHz). We show that this result is not inconsistent with the presence of a component due to non-linear, turbulent-like motions, which, in addition to linear oscillation modes, is shaping the observed spectral distribution.On leave of absence from Institut d'Astrophysique, Cointe-Ougrée, Belgium.     

Electron density diagnostics for solar ultraviolet lines of O V

We determine the electron densities for a range of solar features using new calculations for the Ov line ratio, R=I(761.1)/I(760.4), in conjunction with observational data obtained with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on the Solar and Heliospheric Observatory (SOHO). The densities obtained from this diagnostic are in good agreement with earlier measured values. This provides support for the theoretical diagnostics presented in this paper, and hence the atomic data used in their derivation. We conclude from these results that this particular Ov ratio is a useful diagnostic for many types of solar features.     

The Formation of MgI 4571 Å in the solar atmosphere

The two-dimensional equation of transfer is solved for the case of locally-controlled source function (LTE) and radiationally-controlled ionization. Horizontal fluctuations in electron temperature and macroscopic velocity fields are superposed on the basic one-dimensional model (cf. Altrock and Cannon, 1972). Output intensities are compared with observed rms intensity fluctuations and spatially-averaged intensities in Mg i 4571 Å. We find that at least one model (with a height-independent temperature fluctuation T/T=±0.02 in the range 0h450 km) can predict the magnitude of the intensity fluctuations in both the continuum and 4571 Å. The asymmetry of the line can be explained by adding a height-independent, temperature-correlated flow of amplitude 1 to 2 km s^–1. The relationship between these results and other multi-dimensional analyses is discussed.On leave from Department of Applied Mathematics, University of Sydney, Sydney, Australia.     

The α-Effect and proton acceleration in the solar wind

The quantum field model is used to study the correlation functions of velocity and magnetic fluctuations in helical developed MHD turbulence of solar wind which is generated by random forces with mixed noise correlators. The exponential increase of the magnetic fluctuations is stabilized by spontaneous symmetry breaking mechanism, which leads to the creation of homogeneous magnetic field 〈E〉, and consequently, gives rise to the α-effect. The maximum value of the α-effect is determined in the Kolmogorov universal regime and its contribution to the proton acceleration is estimated. The contribution of the α-effect to ∼100 MeV proton acceleration is discussed and compared with the 2nd Fermi acceleration mechanism. This article was submitted by the authors in English.     

A magnetic core in the Sun? The solar rotator

The previously found solar distortion rotating rigidly and wave-like on the surface with a 12 day period is interpreted as the shape of the gravitational potential induced by the solar core distorted by an internal magnetic field and rotating rigidly with this period. The distortion does not have a symmetry axis and the necessary magnetic field is not compatible with the axial symmetry required of a quasi-static field locked in the rotating core. It is concluded that if the solar distortion is due to such a process the core is oscillating with a very long period, a toroidal oscillation with a period of the order of years.This research was supported in part by the National Science Foundation.     

The Hα/Hβ ratio in solar flares

We have measured the ratio of H to H central intensities in the peak kernels of 14 flares, using simultaneous filtergrams. The ratio is typically one with some scatter. By contrast, in stellar flares the ratio is about 0.8.     

Diagnostics of the solar atmosphere by the Non-LTE inversion method: Line of Ba II λ 455.403 nm

The inverse problem of nonequilibrium radiative transfer in the Ba II λ 455.403 nm line is solved taking into account the hyperfine structure and isotopic splitting. Diagnostic capabilities of this line for the study of the solar atmosphere are investigated based on the three-dimensional hydrodynamic model. It is shown that the use of inversion methods for the analysis of the observed Ba II λ 455.403 nm line makes it possible to reproduce physical conditions in the layers of the photosphere and the lower chromosphere (0 < h < 600 km) with a correlation coefficient of 0.9.     

Stars with discrepant v sin i as derived from Ca II λ 3933 Å and Mg II λ 4481 Å lines. III. Stars with v sin i (λ 3933 Å) < v sin i (λ 4481 Å)

Axial rotation of a star plays an important role in its evolution, physical conditions in its atmosphere and the appearance of its spectrum. We analyzed CCD spectra of nine stars for which the projected rotational velocity derived from the Ca II line at λ 3933 Å was remarkably lower than the one derived from the MgII line at λ 4481 Å. We derived effective temperatures and surface gravities using published uvbyβ photometries, and computed synthetic spectra. Comparing the observed line profiles of the two lines with the computed ones, we estimated the values of v sin i. One of the stars, HD44783, is a Be-star which, besides the narrow absorptions in the spectrum originating in its circumstellar envelope, also has lines of interstellar origin. We also found indications of circumstellar matter in the spectrum of HD25152. In the spectra of the remaining seven stars the narrow components in the Ca II λ 3933 Å line as well as narrow absorptions in the Na I λ 5889.951 Å (D1) and λ 5895.924 Å (D2) lines are of interstellar origin. In HD114376 there are two systems of interstellar components, thus disclosing two different interstellar clouds in the direction of the star. In the spectrum of HD138527 signs of a possible companion were detected, the emission of which contributes 15% to the total light of the system.     

On the centre-to-limb variation and latitude dependence of the asymmetry and wavelength shift of the solar line λ 5576

Low noise photoelectric measurements of the line profile of the g = 0 Fe line gl 5576.097 combined with determinations of the wavelength shift of its centre calibrated by use of an I ₂ absorption tube are reported. Measurements taken at various limb distances (1.0 cos 0.2) and along 4 different diameters of the Sun are used to investigate the behaviour of the line asymmetry (C-shape) and wavelength shift of the line centre as functions of cos and of latitude and to search for possible pole-equator differences.An accuracy of approx. 0.8 mÅ r.m.s. is achieved for the determination of the centre of the solar line relative to the iodine lines and of 0.3 mÅ to 1 mÅ r.m.s. for the relative variations of the C-shape. The analysis shows a significant difference between the limb-effect curves along polar and equatorial diameters for cos 0.4 and changes of the C-shape for 0.9 cos 0.6 with a rather strong indication of a latitude dependence of the C-shape. This latitude dependence may account for the so-called ears observed by Howard et al. (1980) who used the well-known Doppler compensator method which integrates the line asymmetry from the line wings to the core.Mitteilungen aus dem Kiepenheuer-Institut Nr. 207.     

The excitation of the iron Kα feature in solar flares

We evaluate the relationship between the hard X-ray photon spectrum and the flux of iron K emission in a thick-target electron bombardment model. Results are presented for various power-law hard X-ray spectra. We then apply these results to two events observed with the Hard X-Ray Burst Spectrometer and the K channel of the X-Ray Polychromator Bent Crystal Spectrometer on the Solar Maximum Mission satellite. For one of the events, on 29 March, 1980, at 09:18 UT, the K flux predicted for a thick-target non-thermal process is significant compared to the background fluorescent component, and the data are indeed consistent with an enhancement of the predicted amount. For the other event, on 14 October, 1980 at 06:09 UT, the hard X-ray spectrum is so steep that no significant Ka flux is predicted for this process, and no enhancement is seen. We conclude that the agreement between the predicted K flux and the observed magnitude of the K enhancement above the fluorescent background at the time of the large hard X-ray bursts lends support to a thick-target non-thermal interpretation of impulsive hard X-ray emission in solar flares.     

The maximum limit of the density variation parameter λ for a stable neutron star

A static spherically-symmetric model, based on an exact solution of Einstein's equation, gives the permissible matter density 2×10¹⁴ g cm^–3. If we use the change in the ratio of central density to the radiusr=a (i.e., central density per unit radius (₀/a), we call it radius density) minimum, we have estimated the upper limit of the density variation parameter () and minimum mass limit of a superdense star like a neutron star. This limit gives an idea of the domain where the neutron abundance with negligible number of electrons and protons (may be treated as pure neutrons) and equilibrium in neutrons begins.