On 2D radiative transfer in solar granulation. The case of Fe I lines

The effects have been investigated of non-vertical radiative transfer on the formation of neutral iron spectral lines in inhomogeneous photosphere of the Sun. The non-vertical effects, as well as deviations from LTE, smooth the contrast of solar granulation in equivalent widths of solar absorption lines. For the central residual intensities of moderate and strong lines, on the contrary to weak lines, these effects enhance the granulation contrast. It was found, that the negative dependency of equivalent width on intensity in continuous spectrum decays due to the non-vertical effects for Fe I lines.     

The interactions of the solar wind with planetary magnetic fields: Basic principles and observations

A planetary magnetic field obstructs the supersonic expansion of the solar coronol ions and electrons and creates a cigar shaped bubble in the solar wind. The pressure of the solar wind on the bubble compresses and seriously distorts the planetary magnetic field. A review is given here of the theoretical calculation and experimental observation of the shape of the bubble and the configuration of the compressed magnetic field inside the surface. Other effects, namely the shock structure on the surface, the radiation belts, and the current in the antisolar side of the bubble are described as well as a simplified account of electric fields and the connection of the planetary field lines with interplanetary field lines.     

Solar Chemical Abundances Determined with a CO5BOLD 3D Model Atmosphere

In the last decade, the photospheric solar metallicity as determined from spectroscopy experienced a remarkable downward revision. Part of this effect can be attributed to an improvement of atomic data and the inclusion of NLTE computations, but also the use of hydrodynamical model atmospheres seemed to play a role. This “decrease” with time of the metallicity of the solar photosphere increased the disagreement with the results from helioseismology. With a CO ⁵ BOLD 3D model of the solar atmosphere, the CIFIST team at the Paris Observatory re-determined the photospheric solar abundances of several elements, among them C, N, and O. The spectroscopic abundances are obtained by fitting the equivalent width and/or the profile of observed spectral lines with synthetic spectra computed from the 3D model atmosphere. We conclude that the effects of granular fluctuations depend on the characteristics of the individual lines, but are found to be relevant only in a few particular cases. 3D effects are not responsible for the systematic lowering of the solar abundances in recent years. The solar metallicity resulting from this analysis is Z=0.0153, Z/X=0.0209.     

Effects on UV lines observations of stationary plasma flows confined in coronal loops

We describe a method which uses a rather detailed model of coronal loop hosting a siphon flow as a diagnostic tool to interpret solar UV and X-ray observations in selected bands and lines. We apply the method to investigate the deviations from ionization equilibrium induced by stationary plasma flows confined in coronal loops and their effects on the UV and EUV emission lines observed by the instruments on board SOHO. We present results on the detailed synthesis of loop emission in a set of selected emission lines observed by CDS and SUMER, taking into account the non-equilibrium of ionization effects.     

The NLTE Formation of Iron Lines Used in Solar Polarimetry

We study in some detail one-dimensional NLTE effects in solar Fei lines. The lines selected are frequently used in solar polarimetry, and also in studies of line asymmetries and for abundance determinations. Our model atom for Fei–Feii–Feiii is realistic: it takes account of multiplet structure and it includes over 200 bound–bound and bound–free transitions in detail. We use very efficient iterative methods for the self-consistent solution of the kinetic and radiative transfer equations (Auer, Fabiani Bendicho, and Trujillo Bueno, 1994). We have applied these fast methods of solution because they are suitable for the investigation of 2D and 3D NLTE transfer effects with multilevel atoms, which constitutes the next step of our ongoing research project on the iron line formation problem.     

Synthetic high-resolution near-IR spectra of the Sun for planetary data reductions made from ATMOS/Spacelab-3 and Atlas-3 data

We have constructed synthetic solar spectra for the 2302-4800 cm⁻¹ (2.08-4.34 μm) range, a spectral range where planetary objects mainly emit reflected sunlight, using ATMOS (Atmospheric Trace Molecule Spectroscopy)/Spacelab-3 and Atlas-3 spectra, of which resolution is 0.01 cm⁻¹. We adopted Voigt line profiles for the modeling of line shapes based on an atlas of line identifications compiled by Geller [Geller, M., 1992. Key to Identification of Solar Features. A High-Resolution Atlas of the Infrared Spectrum of the Sun and the Earth Atmosphere from Space. NASA Reference Publ. 1224, vol. III. NASA, Washington, DC, pp. 1-22], who derived solar line positions and intensities from contaminated high-resolution solar spectra obtained by ATMOS/Spacelab-3. Because the ATMOS spectra in these wavelength ranges are compromised by absorption lines of molecules existing in Earth's high-altitude atmosphere and in the compartment of the spacecraft, the direct use of these high-resolution solar spectra has been inconvenient for the data reductions of planetary spectra. We compared the synthetic solar spectra with the ATMOS spectra, and obtained satisfactory fits for the majority of the solar lines with the exception of abnormal lines, which do not fit with Voigt line profiles. From the model fits, we were able to determine Voigt line parameters for the majority of solar lines; and we made a list of the abnormal lines. We also constructed telluric-line-free solar spectra by manually eliminating telluric lines from the ATMOS spectra and filling the gaps with adjacent continua. These synthetic solar spectra will be useful to eliminate solar continua from spectra of planetary objects to extract their own intrinsic spectral features.     

The solar photospheric abundance of zirconium

Zirconium (Zr), together with strontium and yttrium, is an important element in the understanding of the Galactic nucleosynthesis. In fact, the triad Sr‐Y‐Zr constitutes the first peak of s‐process elements. Despite its general relevance not many studies of the solar abundance of Zr were conducted. We derive the zirconium abundance in the solar photosphere with the same CO⁵BOLD hydrodynamical model of the solar atmosphere that we previously used to investigate the abundances of C‐N‐O. We review the zirconium lines available in the observed solar spectra and select a sample of lines to determine the zirconium abundance, considering lines of neutral and singly ionised zirconium. We apply different line profile fitting strategies for a reliable analysis of Zr lines that are blended by lines of other elements. The abundance obtained from lines of neutral zirconium is very uncertain because these lines are commonly blended and weak in the solar spectrum. However, we believe that some lines of ionised zirconium are reliable abundance indicators. Restricting the set to Zr II lines, from the CO⁵BOLD 3D model atmosphere we derive A (Zr) = 2.62 ± 0.06, where the quoted error is the RMS line‐to‐line scatter (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Variation in the Width of Transition Region Network Boundaries

The transition region network seen in solar extreme ultraviolet (EUV) lines is the extension of the chromospheric network. The network appears as an irregular web-like pattern over the solar surface outside active regions. The average width of transition region network boundaries is obtained from the two-dimensional autocorrelation function of SOlar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) synoptic images of the Sun in two emission lines, He i 586 Å and O v 630 Å during 1996?–?2012. The width of the network boundaries is found to be roughly correlated with the solar cycle variation with a lag of about ten months. A comparison of the widths in the two emission lines shows that they are larger for the He i line. The SOHO/CDS data also show large asymmetry in boundary widths in the horizontal (x) and vertical (y) image directions, which is shown to be caused by image distortions that are due to instrumental effects. Since the network boundary widths are related to the magnetic flux concentration along the boundaries, the results are expected to have implications on the flux transport on the solar surface, solar cycle, and the mass and energy budget of network loops and jets.     

The Hanle effect and the diagnostics of turbulent magnetic fields in the solar atmosphere

The theory of the Hanle effect is used to interpret the linear polarization measured in a number of spectral lines on the solar disk near the heliographic north and south poles, in search for a turbulent magnetic field in the solar atmosphere. The Hanle depolarization is separated from a number of other effects, including collisional depolarization and scattering geometry. Although the main aim of the paper is to elucidate the physics of the Hanle effect as applied to the Sun, our results indicate the existence of hidden or turbulent magnetic flux near the temperature minimum of the solar atmosphere, with a field strength between 10 and 100 G. This field is hidden in the sense that it is not seen in measurements of the longitudinal Zeeman effect (solar magnetograms). It carries more total magnetic flux than the kG network fields.     

Isotopes of samarium in the sun

Terrestrial samarium consists of seven isotopes. Some spectral lines from Sm have isotope shifts and hyperfine structures that will modify the profile of the absorption lines in the Fraunhofer spectrum. The photospheric spectrum around the Sm ii lines at 4467 and 4519 Å has been studied. Although it is impossible to derive the solar abundance of each individual isotope, it is shown that a terrestrial isotopic composition can account for the anomal line width and asymmetry of the observed solar lines. The solar abundance found from the two lines is A(Sm) = 1.54 in the logarithmic A(H) = 12.00 scale.     

Polarity neutral lines on the solar surface and magnetic structures in the corona

This paper elucidates the topological relationship between the distribution of polarity neutral lines on the solar surface and the interspersion of closed field lines among open field lines in the corona. The solar surface contains polarity neutral lines, that are spatially nested in a series-and-parallel hierarchy. The corona is partitioned by separatrix surfaces into a corresponding hierarchy of nested magnetic cells. The complexity of the magnetic structure of the corona consists in the embedding of magnetic cells of closed field lines amid open field lines.Polarity neutral lines lie necessarily on the foot surfaces of magnetic cells that are filled with closed field lines. There are two topologically distinct types of magnetic cells of closed field lines: closed and open. Only the open cells are overlain by current sheets. Each of the heliospheric current sheets separates the open field lines encircled by an open cells from the open field lines encircling the cell. Since closed cells have no images in the outer corona, the cell structure of the latter reflects those polarity neutral lines associated with the open cells in the lower corona. Accordingly, there are fewer heliospheric current sheets, as revealed by magnetic neutral lines on the source surface, in interplanetary space than polarity neutral lines on the solar surface.     

Oscillator strengths for selected Fe II lines in the range λλ 300–400 nm

We review available published data on the oscillator strengths determined for ionized iron Fe II lines. Particular attention is given to the lines in the ultraviolet spectrum. The oscillator strengths are calculated for 33 Fe II lines observed in the solar spectrum between λ 300 and 400 nm. The results are derived by fitting our data to the solar equivalent widths.     

Mass motion in the solar chromosphere

A high resolution profile of the solar Oi 1304.9 Å line has been measured from rocket spectrograms. The profile is nearly flat-topped, showing only a slight solar reversal after instrument effects and absorption due to atomic oxygen in the earth's atmosphere have been allowed for. A theoretical analysis of this line, under the assumption of non-LTE conditions and a homogeneous, spherically symmetric chromosphere, predicts a rather deep solar reversal. The theoretical profile may be made consistent with the observed profile if mass motion is present in the chromospheric region where the line is formed. A Gaussian distribution of up and down velocities with a root mean square velocity of about 7 km/sec gives best agreement between the predicted and observed profile. This result is consistent with the conclusion made from a study of high resolution profiles of solar lines in the visible spectrum that mass vertical velocities increase with height above the photosphere.     

Spectroscopic Coronal Observations During the Total Solar Eclipse of 11 July 2010

The flash spectra of the solar chromosphere and corona were measured with a slitless spectrograph before, after, and during the totality of the solar eclipse of 11 July 2010, at Easter Island, Chile. This eclipse took place at the beginning of Solar Cycle 24, after an extended minimum of solar activity. The spectra taken during the eclipse show a different intensity ratio of the red and green coronal lines compared with those taken during the total solar eclipse of 1 August 2008, which took place toward the end of Solar Cycle 23. The characteristic coronal emission line of forbidden Fe xiv (5303 Å) was observed on the east and west solar limbs in four areas relatively symmetrically located with respect to the solar rotation axis. Subtraction of the continuum flash-spectrum background led to the identification of several extremely weak emission lines, including forbidden Ca xv (5694 Å), which is normally detected only in regions of very high excitation, e.g., during flares or above large sunspots. The height of the chromosphere was measured spectrophotometrically, using spectral lines from light elements and compared with the equivalent height of the lower chromosphere measured using spectral lines from heavy elements.     

Clarification in the identification of certain lines in the infrared solar spectrum

Forty-two unidentified spectral lines in Mohler's (1955) table of infrared solar spectrum wavelengths are shown to be solar, and three of these tentatively have been identified. One line, listed by Mohler as solar or telluric, is shown to be telluric.     

The solar abundance of calcium and collision broadening of Ca i- and Ca ii-Fraunhofer lines by hydrogen

An abundance analysis of the solar calcium spectrum is carried out using 46 lines with known f-values in the visible and near infrared spectral region. Resonance, forbidden and autoionizing lines are included. The solar abundance of calcium resulting from the 25 weaker, nearly damping-independent lines only is log _Ca=6.36±0.07, on the scale log H = 12. The great variety of transitions involved in the solar calcium spectrum, ranging from 0 eV lines of CaI to 7.5 eV lines of CaII and including autoionizing lines, are in reasonable agreement (Figure 1b). Therefore notable non-LTE effects on their equivalent widths can be excluded.Together with the sodium abundance log _Na = 6.30 determined earlier, the solar abundance ratio Ca/Na = 1.15 is obtained with an accuracy of 10%. Comparison with meteorites (carbonaceous chondrites I) shows that solar and meteoritic ratio agree within these limits.The line broadening by collisions with hydrogen atoms is determined empirically from a comparison of weak and strong Fraunhofer lines of CaI and CaII, thereby using the solar atmosphere as an absorption tube of comparatively well-known physical state. The damping half-widths H turn out to be larger than predicted from pure van der Waals interaction, the average enhancement factor being 3.0 for CaI and 1.7 for CaII, independent of term properties to a first approximation. Regardless of the reason for this enhancement — inaccurate van der Waals theory or predominance of repulsive interaction these results can be used in the spectroscopy of other stars.     

Solar corona photometry by electropolarimetric and CCD observations during the eclipse of July 11, 1991

The observational data of the solar corona obtained during the solar eclipse of July 11, 1993, using both a electropolarimeter (EP) and a CCD matrix were processed. Using these data the photometry of the solar corona was carried out. The results of EP data were compared with those of the CCD data. It must be noticed here that the CCD data give us only the characteristics of the inner corona, while the EP data show the features of both the inner and the middle corona up to 4 solar radii. The standard flattening index ϵ was evaluated from both data. The dependence of ϵ on the distance from the solar limb was investigated. Three-dimensional images of the coronal intensity distribution for different spectral lines are shown. Isophotes in Na and Ca lines with unusual features are plotted. Based on these data some ideas and conclusions on the type of the solar corona and the physical conditions in it are presented.     

Multiparameter Computations of Solar Wind Characteristics in the Near-Earth Space from the Data on the Solar Magnetic Field

The solar wind parameters were analyzed using the concept which is being developed by the authors and assumes the existence of several systems of magnetic fields of different scales on the Sun. It was demonstrated that the simplest model with one source surface and a radial expansion does not describe the characteristics of the quiet solar wind adequately. Different magnetic field subsystems on the Sun affect the characteristics of the solar wind plasma in a different way, even changing the sign of correlation. New multiparameter schemes were developed to compute the velocity and the magnetic field components of the solar wind. The radial component of the magnetic field in the solar corona and the tilt of the heliospheric current sheet, which determines the degree of divergence of field lines in the heliosphere, were taken into account when calculating the magnetic field in the solar wind. Both the divergence of field lines in the corona and the strength of the solar magnetic field are allowed for in calculating the solar wind speed. The suggested schemes provide a considerably higher computation accuracy than that given by commonly used one-parameter models.     

Extreme vacuum ultraviolet solar spectra obtained during the SPIRIT experiment aboard CORONAS-F: A catalog of lines in the range 280–330 Å

We present a catalog of 100 lines in the wavelength range 280–330 Å detected by the RES-C spectroheliograph in solar active regions and flares during the SPIRIT experiment aboard the CORONAS-F orbital station. We identified 54 lines. The line intensities recorded during the X3.4 (GOES) solar flare of December 28, 2001, are given. The data reduction procedure is discussed.