Recent three-dimensional radiation hydrodynamic simulations by Wedemeyer et al. 2004 suggest that the solar chromosphere is highly structured in space and time on scales of only 1000 km and 20-25 sec, resp.. The resulting pattern consists of a network of hot gas and enclosed cool regions which are due to the propagation and interaction of shock fronts. In contrast to many other diagnostics, the radio continuum at millimeter wavelengths is formed in LTE, and provides a rather direct measure of the thermal structure. It thus facilitates the comparison between numerical model and observation. While the involved time and length scales are not accessible with todays equipment for that wavelength range, the next generation of instruments, such as the Atacama Large Millimeter Array (ALMA), will provide a big step towards the required resolution. Here we present results of radiative transfer calculations at mm and sub-mm wavelengths with emphasis on spatial and temporal resolution which are crucial for the ongoing discussion about the chromospheric temperature structure.

Three-dimensional numerical simulations with (COBOLD)-B-5, a new radiation hydrodynamics code, result in a dynamic, thermally bifurcated model of the non-magnetic chromosphere of the quiet Sun. The 3D model includes the middle and low chromosphere, the photosphere, and the top of the convection zone, where acoustic waves are excited by convective motions. While the waves propagate upwards, they steepen into shocks, dissipate, and deposit their mechanical energy as heat in the chromosphere. Our numerical simulations show for the first time a complex 3D structure of the chromospheric layers, formed by the interaction of shock waves. Horizontal temperature cross-sections of the model chromosphere exhibit a network of hot filaments and enclosed cool regions. The horizontal pattern evolves on short time-scales of the order of typically 20-25 s, and has spatial scales comparable to those of the underlying granulation. The resulting thermal bifurcation, i.e., the co-existence of cold and hot regions, provides temperatures high enough to produce the observed chromospheric UV emission and - at the same time - temperatures cold enough to allow the formation of molecules (e.g., carbon monoxide). Our 3D model corroborates the finding by Carlsson & Stein (1994) that the chromospheric temperature rise of semi-empirical models does not necessarily imply an increase in the average gas temperature but can be explained by the presence of substantial spatial and temporal temperature inhomogeneities.

Based on high S/N spectra obtained at La Silla, Chile, and the Special Astrophysical Observatory, Russia, the abundances of He, C, O, Ne, Mg, Si, Ca, Fe, Sr, and Ba in 27 optically bright B5-B9 main-sequence stars were determined. NLTE effects were taken into account. A variety of abundance patterns is present in late B stars. Accurate surface abundances of the diffusion indicators O, Mg, Ca, Sr and Ba suggest that element stratification due to diffusion is common in the program stars. Models of stellar atmospheres which include meridional mixing can explain the observed anomalies. Although the program stars represent only a volume-limited sample of the solar neighbourhood this result is important for the cosmochemical evolution of the Galaxy: the surface abundances of the stars investigated do not necessarily reflect the chemical composition of the interstellar cloud they originated from. Furthermore, five program stars show narrow absorption lines in Ca II K which can be attributed to circumstellar gas. Neon serves as a trace element for the occurrence of weak stellar winds. Neon overabundances of some stars derived under the assumption of LTE suggest that such winds have been detected. In sharp contrast, the more realistic treatment of NLTE leads to solar neon abundances and thus reveals that weak stellar winds are absent in the program stars.

I01 Numerical Models of the Structure of the Solar Tachocline I02 What can we learn from Local Convection Simulations in the Context of Mean Field Models of Stellar Rotation and Magnetism? I03 Anelastic Simulations of Convection and Magnetic Fields in Cartesian Geometry I04 Helicity and Reynolds Stresses in Convection Zone with NIRVANA I05 The Kelvin-Helmholtz and Shear Instabilities of a Vortex Flow Around a Magnetic Flux Tube I06 Convection and Small-scale Magnetic Fields in M-type Atmospheres I07 Modelling the Chromospheric Background Pattern of the Non-magnetic Sun I08 A Numerical Investigation on the Role of the Magnetorotational Instability in Galactic Disks I09 Turbulence Due to the Shear Instability in Stratified Galactic Disks I10 Outflows and Accretion in a Protostellar Star-disc System I11 Time-dependent ADAF-SSD Transition Models I12 A 5D Numerical Method for Exploring the Magnetic Field Topology in Jet Environs I13 Chaos and Order in Three-Dimensional Magnetic Reconnection I14 Theory of Magnetic Pinching of Hyperbolic Flux Tubes I15 Extension of Taylor's Principle to Plasma-vacuum Systems I16 Formation of Current Sheets and Sigmoidal Structure by the Ideal Kink Instability of a Magnetic Loop I17 On Stationary Resistive MHD Flows with Application to the Heliosphere I18 A Biot-SavartMethod to Handle Time-dependent Dynamos in Arbitrary Domains I19 Complete Numerical Solution of the MHD and Hall-MHD Structure Problem I20 Convergence of a Solenoidal Discrete Rot-operator I21 Instabilities of the MC Waves Type in the Earth's Core I22 A Shallow Water Model for the Liquid Metal Pinch I23 DNS of Transitional and Turbulent Flow Driven by a Rotating Magnetic Field I24 New Results in Riga Dynamo Experiment I25 The Screw Dynamo in Torus Geometry

We determined abundances of O, Ca, Fe, Ba and Y for a sample of dusty and dust-free A stars, taken from the list of Cheng et al. (1992). Five of the stars have an infrared-excess due to circumstellar dust. Ongoing accretion from their circumstellar surroundings might have modified the abundances in the photospheres of these stars, but our results clearly show, that there is no difference in the photospheric composition of the dusty and dust-free stars. Instead all of them show the typical diffusion pattern which diminishes towards larger rotational velocities.

In an effort to estimate the largely unknown effects of photospheric temperature fluctuations on spectroscopic abundance determinations, we have studied the problem of LTE line formation in the inhomogeneous solar photosphere based on detailed 2-dimensional radiation hydrodynamics simulations of the convective surface layers of the Sun. By means of a strictly differential 1D/2D comparison of the emergent equivalent widths, we have derived "granulation abundance corrections" for individual lines, which have to be applied to standard abundance determinations based on homogeneous 1D model atmospheres in order to correct for the influence of the photospheric temperature fluctuations. In general, we find a line strengthening in the presence of temperature inhomogeneities as a consequence of the non-linear temperature dependence of the line opacity. The resulting corrections are negligible for lines with an excitation potential around E-i = 5 eV, regardless of element and ionization stage. Moderate granulation effects (Delta(gran) approximate to -0.1 dex) are obtained for weak, high-excitation lines (E-i greater than or similar to 10 eV) of C I, N I, O I as well as Mg II and Si II. The largest corrections are found for ground state lines (E-i = 0 eV) of neutral atoms with an ionization potential between 6 and 8 eV like Mg I, Ca I, Ti I, Fe I, amounting to Delta(gran) approximate to -0.3 dex in the case of Ti I. For many lines of practical relevance, the magnitude of the abundance correction may be estimated from interpolation in the tables and graphs provided with this paper. The application of abundance corrections may often be an acceptable alternative to a detailed fitting of individual line profiles based on hydrodynamical simulations. The present study should be helpful in providing upper bounds for possible errors of spectroscopic abundance analyses, and for identifying spectral lines which are least sensitive to the influence of photospheric temperature inhomogeneities.

In an effort to estimate the largely unknown effects of photospheric temperature fluctuations on spectroscopic abundance determinations, we have studied the problem of LTE line formation in the inhomogeneous solar photosphere based on detailed 2-dimensional radiation hydrodynamics simulations of the convective surface layers of the Sun. By means of a strictly differential 1D/2D comparison of the emergent equivalent widths, we have derived “granulation abundance corrections” for individual lines, which have to be applied to standard abundance determinations based on homogeneous 1D model atmospheres in order to correct for the influence of the photospheric temperature fluctuations. In general, we find a line strengthening in the presence of temperature inhomogeneities as a consequence of the non-linear temperature dependence of the line opacity. The resulting corrections are negligible for lines with an excitation potential around Ei=5 eV, regardless of element and ionization stage. Moderate granulation effects ( dex) are obtained for weak, high-excitation lines ( eV) of C i, N i, O i as well as Mg ii and Si ii. The largest corrections are found for ground state lines (Ei=0 eV) of neutral atoms with an ionization potential between 6 and 8 eV like Mg i, Ca i, Ti i, Fe i, amounting to dex in the case of Ti i. For many lines of practical relevance, the magnitude of the abundance correction may be estimated from interpolation in the tables and graphs provided with this paper. The application of abundance corrections may often be an acceptable alternative to a detailed fitting of individual line profiles based on hydrodynamical simulations. The present study should be helpful in providing upper bounds for possible errors of spectroscopic abundance analyses, and for identifying spectral lines which are least sensitive to the influence of photospheric temperature inhomogeneities.

Current problems encountered in the spectroscopic determination of photospheric abundances are outlined and exemplified in a reevaluation of C, N, O, Ne, Mg, Si, and Fe, taking effects of NLTE and granulation into account. Updated abundances of these elements are given in Table 2. Specific topics addressed are (1) the correlation between photospheric matter and CI chondrites, and the condensation temperature below which it breaks down (Figure 1), (2) the question whether the metallicity of the Sun is typical for its age and position in the Galaxy.

This is the rapporteur paper of Working Group 2 on Measuring Solar Abundances. The working group presented and discussed the different observations and methods for obtaining the elemental and isotopic composition of the Sun, and critically reviewed their results and the accuracies thereof. Furthermore, a few important yet unanswered questions were identified, and the potential of future missions to provide answers was assessed.

We analyze the Fe/O elemental abundance ratio in the solar wind from SOHO/CELIAS/CTOF data. Analyzed in different solar wind regimes this ratio is indicative of the strength of the FIP fractionation process because iron is a low FIP element and oxygen is a high FIP element. It is investigated whether there is a significant fractionation of the Fe/O ratio in the coronal hole solar wind. For the first time, to our knowledge, we attempt to eliminate the influence of model-dependent parameters on the derivation of the photospheric Fe/O ratio thereby increasing the accuracy of its determination. The Fe/O ratio in coronal hole solar wind is slightly higher than in the photosphere. Even with our optimistic estimates of the uncertainties of the photospheric Fe/O ratio the observed fractionation is only marginally significant.